I'm not positive about this, but I think it also has something to do with the higher rate of asexual reproduction in plants. Accidental polyploidy in an animal species is likely to render that individual infertile, and unless the animal is capable of reproducing through parthenogenesis, they will not be able to pass the polyploid condition on to any offspring. Plants are more likely to be able to create 'clones' through budding, fragmentation, or other types of asexual reproduction, and so more polyploid individuals may be created. This might allow the plant in question to undergo other processes that could make it fertile again (chromosomal rearrangements of some sort).
Of course, this doesn't begin to cover the fact that Cypress brought up - that polyploid animals are formed but usually do not survive to maturity. So obviously there is more to it than that, but I thought I'd throw it out there.