Why did my boarding pass have to be manually checked at the gate? [closed]

I recently travelled from the US to the UK with my friends for a trip, during boarding, my boarding pass and one of my friend's boarding pass was rejected by the scanner at the gate during boarding and we had to step aside while they manually checked our boarding passes and passports. On the flight back to the US, the same thing happened again but only to my boarding pass this time and it took them longer to manually check it this time. What could've possibly happened to set off the scanner?