I have hazy recollections of hearing this a long time ago, and I vaguely recall that it was ostensibly to minimise the risk - during the transition from the "dropping like a brick through the breeze" phase to the "floating like a cloud on the breeze" phase - of only (un)stalling one of the sticky-out bits and somehow popping into a spin rather than flying out of a stalling. Can't for the life of me remember what the underlying mechanics were meant to be to achieve this, tho
Wasn't particularly convinced then, and no more so now.
ETA:
Originally Posted by wantobe
Entering the stall in a cross wind situation (especially in a stronger wind) makes it difficult for them to keep the aircraft straight and in balance (weather cock). Usually resulting in a wind drop stall.
Actually, that rings a bell. That might very well have been the "reason".