Apparently it's when it occurs in the great state of Utah.

http://wire.jacksonville.com/pstories/sports/college/20040610/2229305.shtml

I've been to BYU before...that Y is HUGE, and is the very symbol of the college. That cost a crapload of money to fix.
When is a prank a velony? when it costs over 10 grand to fix. its not like they are being charged for TPing the BYU Mess hall that costs 13 bucks to fix.