Probably the same thing that cuases the 6-8 legged frogs, that virus that causes thier cells to start over durring development.

Originally posted by Catfish
Looks like 3 frogs on top of eachother to me..
The BBC news item, Puzzle over three-headed frog (originally titled "'Warning' over three-headed frog") spawned this story that swept the news media and the weblog circuit over the week following March 5th 2004. Briefly: staff and pre-school children at the Green Umbrella day nursery, Weston-super-Mare, UK, found the above. After they'd taken photos and a video, it escaped. The BBC took up the story, citing one of their own wildife experts, biologist and presenter Mike Dilger, as "stunned" and saying "it could be an early warning of environmental problems" (they published the same factoid in their CBBC Newsround children's section). From there, the tale snowballed to newspapers worldwide.