Find what caused a current big flight delay?

The airline always knows the reason for its flight to be delayed. In practice, of course, this means only some people know that, and the agent you're talking to might not be one of them. So your quest is to find the agent who a) knows this and b) is authorized to share this information.

Generally, in terms of knowledge, based on my personal experience:

• Gate agent usually knows most up-to-date information - but can only be reached at gate;
• Check-in agent might know some information (they generally handle check-ins for multiple flights, so they are not up to date);
• Airline service desk at the airport can find out the up-to-date information by calling the gate agent, if you are nice to them (they don't really have to);
• Airline service phone desk also might find out this for you. This is extra hassle for them, as they have to find out the direct phone, call it, and transfer this information. The chance for them to do it increases if you're nice, if the workload is relatively low, or if you're a premium cabin or high status passenger.
• Asking in public channel (i.e. Twitter) seems to offer a better chance to receive the response, as the airlines tend to care about their public image

Note that the reason for the flight delay might change in time. For example what started as an aircraft requiring maintenance could change to weather delay, or (in case of next day) strike delay.

What you are really asking is "Does the fact that the flight was delayed today increase the chance it will be delayed tomorrow?". Finding out the reason can give information on that question, but rarely. It depends on the type of flight. There are some long haul flights where the same aircraft makes one round trip per day. If there was a long delay today, you may not have looked late enough to find out today's flight was canceled entirely because of a problem with the plane. That might well carry on until tomorrow. Even if it doesn't, tomorrow's flight will want to accommodate today's passengers, so some people will be bumped. It could be you. In this scenario, it would increase my estimate of the chance I will not get out tomorrow.

For shorter haul flights there is more flexibility, but less than you might guess. The airlines do not have many spare aircraft around, but work hard to cover the flight schedule. Many of the airline app sites have "where is this plane coming from?" buttons that can let you track whether they intended to service your flight with the same plane that was delayed today. What you don't know is whether they will reassign aircraft to flights because of the delay.

My experience is that the information you will get on the reason for the delay today (assuming you get any at all) will not be detailed enough to make a reasonable assessment for the impact on tomorrow. It is natural to think flight XXX was delayed today, so there is a higher chance of it being delayed tomorrow, but it is very hard to say whether that is a rational assessment. On the other hand I have seen airlines very slow to admit that one problem will propagate to future problems. I have had them claiming that a flight will leave on time when the incoming aircraft had not left the previous airport in time to support that claim.

The next question is what are you going to do with the information? If you didn't know today's flight was delayed you would go to the airport tomorrow and try to get on the plane. Are you now going to cancel that flight and try to buy some other ticket? Are you going to cancel the trip entirely? I don't know your situation, but probably the best thing is to go to the airport tomorrow and see what happens. Good luck.

The odds of a mechanical issue in one aircraft today impacting the same flight number tomorrow are pretty small*. The odds of a large weather disruption delaying today's flight and having a knock-on impact on the next day or two or three of flights is pretty large. Finding out about weather related delays is pretty easy - pull up any news or weather site on the internet - you'll read about it. Finding out about mechanical or flight-crew related delays is more difficult, was detailed in the other answers, and probably won't impact future flights.

If your real question is "Will my flight XXnnn be delayed tomorrow?", then on the very page you linked is a history of the last 15 flights under that flight number. If you click on each one in turn, you'll see whether the flight departed early or late and if it arrived early or late. (The 2 Sept, 10:24am departure left late but arrived early - thank you tailwinds!)

If you don't like having to click through each individual flight, FlightRadar24's view of UA48 will only give you 7 days worth, but it will show on one screen when the scheduled and actual departure & arrival times were.

Seven to 14 days of data will probably give you a much better predictor of the recent on-time departure/arrival performance than "yesterday's" performance data. i.e. the one diversion to YYR, return to EWR, then "redeparture" to BOM hasn't happened before or after the incident flight and is a one-off.

*FR24 shows the tail number, and UA hasn't used the same aircraft for any of the last 7 flights completed, nor are they planning to do so for either of the two upcoming flights for which FR24 is showing a predicted tail number. (Note that the actual aircraft to be used is subject to mechanical fitness, etc. and can change - even after the plane has pushed back from the gate.)