Apparently Pythagoras forbid his followers from eating or even touching legumes. There is also the story about Pythagoras being murdered because he refused to escape through a bean field. What did he have against legumes?
We don't know for sure. It wasn't legumes generally, though, it was specifically fava beans.
Even our ancient sources on Pythagoreanism don't know the reason for the taboo. Like Aristotle (in Diog. Laert. 8.34):
According to Aristotle in his work On the Pythagoreans, Pythagoras counselled abstinence from beans either because they are like the genitals, or because they are like the gates of Hades... as being alone unjointed, or because they are injurious, or because they are like the form of the universe, or because they belong to oligarchy, since they are used in election by lot.
Nowadays the bean thing is most often understood as an aspect of Pythagorean teaching on reincarnation, or metempsychosis. A number of non-Pythagorean sources report that beans or bean blossoms are either a route for souls to return to earth, or that souls come from plants, or that beans are like the gates of Hades, or even that the souls of the dead are in beans. In the Roman Lemuria festival, beans were used in a ritual to expel dead spirits from the house. So it seems likely that some aspect of this lies behind the Pythagorean doctrine. However, the reason for the belief itself is unknown; we just don't have the evidence.
A couple of passages suggest that the belief is also tied to the Eleusinian-Orphic mystery religions. Pausanias reports on a temple dedicated to someone called Kyamites, or "beaner", as follows (1.37.4):
On the road stands a small temple called that of Kyamites. I cannot state for certain whether he was the first to sow beans, or whether they gave this name to a hero because they aren't allowed to attribute the invention of beans to Demeter. Whoever has witnessed an initiation at Eleusis, or read the so-called Orphic writings, knows what I mean.
(Unfortunately it will never be possible to do either of those things ever again.) And a few sources report a line from an Orphic poem that states, "eating fava beans is equivalent to eating your parents' heads".
For more detailed discussion and references, the best place to go is Lore and Science in Ancient Pythagoreanism (1972), pp. 180-5, by the sadly missed great scholar Walter Burkert.
Is there any sources on Pythagoras hating legumes? I always assumed his avoidance of beans was out of respect or reverence.