Why do plants produce so many hybrids? I have read that they are the largest kingdom of organisms to do so. Does this have something to do with polyploidy events?
Yes, it has very much to do with the poliploidy events. Specifically if you look at the grasses, most grasses that are diploid (eg., AA, BB), cannot hybridize, but in some rare events when they form polyploid (AAAA, BBBB), they can readily hybridize to form fertile progenies (AABB).