Airdrop Directional File Flow
If it doesn't happen again, you'll probably never know. That's unfortunately the reality of things.
AirDrop, as much other software today, is based on several very complex subsystems. Bugs happen and it can be difficult or almost impossible to explain them or locate them, unless it is possible to reliably reproduce the problem.
It is very likely that some bug caused some software that AirDrop depends on to be in the wrong state. It could be a driver failure, a bug in the network protocol implementation, or any other bug. When you rebooted the Mac, everything starts over from a valid state - and you won't experience the problem.