Because they're epileptic. Tumbler pigeons are bred deliberately for their epilepsy, a form of seizure that makes them tumble and twist as they fly. Apparently if you see a group of them doing it in unison it's very impressive. Severely affected pigeons do not survive very long, but careful management of the breed perpetuates moderate seizures.

I beleive it is because they are epileptic and they have a momentary seisure and fall a bit... could be completely wrong of course but its what I was told...