The pineapple was kind of a running joke at the West Term II (also Term I?) dances. It started at the first dance, themed "Hawaii". Obviously, a pineapple would be a wise choice to hang above the door into K.
Note: A pineapple belonging to one Sean Bibby made an appearance at East II 2010. It was left in Brown Commons one night and was used to play a rousing game of "Hot Potato... er, Pineapple" before it returned to its owner.