What's happening with the bunnies?

I first observed it at my daughter's house in the St. Anthony Park area of St. Paul -- a rabbit with a growth on top of and in the middle of its head. Last week I saw several others with growths of various sizes at Springbrook where this and the next two pictures were taken.

At my house, in Lexington, we have tons of rabbits, but they all appear healthy. Does anyone have any explanation of this growth phenomenon?