In theatre, everything is potentially deadly, or at least could cause grevious bodily harm. Therefor, it is very important to tell everyone exactly what you're doing when you do it. And Everyone has to respond that they hear you. Thus, we yell "THANK YOU" alot.

Example: Flying in a pipe? "PIPE 14 FLYING IN MIDSTAGE" and everyone yells back "THANK YOU PIPE 14"

Turning off the work lights? "WORKS GOING OUT" "THANK YOU WORKS"

yes, this does lead to us also yelling THANK YOU outside of theatre, usually to some confusion. :U