Some of it may be that airplanes induce anxiety peeing.

1) I am in the window seat, everyone else got up to go, this is my only chance. I must go now.
2) The food cart is going to prevent me from using the restroom for next 12 days. I must go now.
3) The plane is going to land and it will take six hours to deplane. I must go now.

In these situations, one may not really HAVE to go so the pressure behind the stream may not be in full force. I don't think this can be avoided.