Okay, seriously Jackson, what the heck?
A PPC curse similar to 'Flaming Denethor!' and 'Jadis in a block of ice!', 'Radagast on a bunny sled!' is used as an exclamation of dismay. It was coined by Lily Winterwood.
Similarly to 'Thranduil on a moose!', 'Radagast on a bunny sled!' is a reference to the decision to stick Radagast the Brown on a sled pulled by giant bunnies in Peter Jackson's movie adaptation of The Hobbit.
Many fans found this rather... bemusing.