The animals believe that Snowball is the one who destroyed the windmills. This is because Napoleon blames him for all the things that go wrong.
I do not agree with them because it was destroyed by a violent November storm that blew it down. Instead of admitting that the windmill's walls were not thick enough to support it against a strong wind, Napoleon blames Snowball for blowing it up, because Snowball had drawn up the plans, the blame for its failure is partly his.