[Vikesrock]

I've never used mecanums and I'm far from an expert, but I believe I remember reading this results from configuring the wheels incorrectly. I can't see what ways the rollers point in the pictures, but I believe the desired configuration is an X and a configuration in the shape of an O causes the behavior you are describing.

I'm sure someone that has used mecanums will be around to either confirm or deny this shortly.