why do the flaps retract when you turn off the engines

I also saw the flaps retract without reason today, but can't reproduce it at will. Can you recall the specific sequence of events which cause it to happen? Just wondering if switching back and forth between the overhead panel/main panel and the control stand causes the flap lever to reset to zero?Cheers.Ian.

flaps 30 after landing turn off engines flaps retract to 0 it takes a little time due to the engine hyd pumps off