Why would an airline put 15 passengers at once on standby?

They may have been non-revenue travelers. As a job perk, airlines commonly allow their own employees to travel for free or at nominal cost (e.g. paying only the taxes), even for personal travel. This privilege may also be extended to the employee's family or friends accompanying them, or to employees of other airlines. But all such travel will typically be standby, so that it does not displace paying customers.

If so, then it's not overbooking, because those people were never promised a seat in the first place. They wait until the last minute, and if seats are still available, they get to board. If not, they can try again on another flight, or maybe give up and go home. They knew all along this was part of the deal. If they really had to get there, they could pay full price for a ticket like everyone else.

Delta domestic flights generally have 3 classes of service - Economy, Economy Comfort and First class.

Passengers with Frequently Flyer "status" are given complimentary upgrades between these cabins, so someone that booked Economy might be upgraded to either First or Economy Comfort. These upgrades sometimes happen in advance, but frequently also happen at the gate.

Based on what you've said, there were 15 people - most likely all of them in Economy - waiting for seats. A small number of these might have been due to the airline "overselling" the flight, but the majority were likely due to free seats being available in First and Economy Comfort, and passengers not having been upgraded to them yet.

Before boarding, the gate staff will "clear" upgrades, moving passengers from Economy Comfort to First, and from Economy to either Economy Comfort or First, in doing so freeing seats in economy for passengers like yourself.

If the flight was oversold then they will attempt to find passengers who are booked on the flight but who will clearly not make their connection (eg, their inbound flight is delayed until after this flight is due to depart), and then allocate those seats as required.

If there are still people without seats they will follow their standard procedure for an oversold flight, which depending on the exact situation could involve asking for 'volunteers' to take a later flight (in return for compensation), or selecting passengers to "Deny Boarding" - although normally this would only happen very close to departure to see if there are any other passengers who fail to show for the flight.

Airlines know very well how many non-shows they have on average for a specific flight, and how many non-revenue passengers.

Using those numbers they can predict quite accurately how many passengers they can safely overbook and put on standby and still not have to leave any paying customers stranded.

Most extreme example I ever had of that was a 747 from Curacao to Amsterdam. That leg was on average 50% overbooked, as it was this time. They still did not have to leave anyone behind, and the aircraft wasn't quite full (but close) on departure. The number of no-shows was that high. On that particular leg at the time this was in large part because of the extremely high number of drugs couriers that use the route. Many get cold feet at the last moment and abandon their journey, more get caught by immigration and customs and are offloaded.

KLM took no great risk when overbooking the flight by 50%, it was routine at the time for that leg (don't know what the numbers are now, probably lower as it's no longer a direct flight and therefore less attractive to drugs couriers, especially those ingesting capsules).

Among other reasons, airlines will refrain from assigning you a seat when all of the remaining unassigned seats are "premium" of any sort - extra leg room, exit row, etc. This is because they're hoping to sell your seat as an upgrade to someone else who's willing to pay for it while waiting at the gate. See my previous answer to a similar question:

Flying AA domestic recently, I was unable to select a seat online without paying extra, and did not receive a proper boarding pass but a "see attendant at gate" note when I checked in at the self-check-in station at the airport. I was worried I'd been bumped, but when I got to the gate they asked me if I was able and willing to assist in the event of an emergency. :-) :-) :-)

(Question originally stated he was on the "Standby" list, but was since changed to "Awaiting Seats" list which makes this answer wrong - but I'm going to leave it for posterity... This answer does NOT answer the question as edited)

You appear to be confusing two different concepts.

You were NOT on standby. You had a confirmed reservation, and your name should not have been shown on the "standby" list. (Some airlines have a separate "awaiting seat allocation" list, but this is separate to the standby list).

However at the time you checked in, they did not have a free physical seat to allocate to you. This could have been due to an oversold situation as you suggested, or it could be simply that they were planning to upgrade one or more other people from Economy to Economy Comfort or First Class, thus freeing a seat for you in economy.

The reason people were on the standby list is because they put themselves on it! There's a number of reasons people could be on the standby list, but the most common are :

• People who are currently booked on a later flight, but who are looking to get on an earlier flight if there are seats available
• Airline staff (or potentially their family/friends) travelling as NRSA (Non-Revenue Space Available) passengers who will be able to get on the flight only if there are seats available.
• People who have purchased "standby" tickets. In general these don't exist on most airlines any more - and I'm fairly sure Delta do not do them.

At times it's possible to see dozens of people on a standby list - especially when there have been flight cancellations/delays/etc. For example, storms in Texas this week caused hundreds of flights in/out of the Houston area to be cancelled. As a result the standby list on my flight from Austin to San Francisco was over 100 people long - all people who would have been rebooked on later flights (potentially the next day) trying to get on something earlier.

The passengers may have been rebooked onto your flight due to a delay or cancellation of their original flight.