No one is really sure to this day what was behind the prohibition. They defended that secret onto their deaths. If I recall correctly there is a story of king Dionysius' soldiers chasing the pythagoreans and only catching them because they refused to cross a field of beans.

The soldiers killed all but the leader of the little group and his pregnant wife. The king had the pregnant pythagorean tortured in an attempt to gain the secrets. In stead of submitting or risking submitting she bit off her own toungue.

So yah . . . they were weird about the beans