I was just going to say 'cos cockers are disgusting little mingers! but this sounds more sensible!!
It's in their DNA. A dead animal means danger around and they roll to disguise their own smell plus they love it!!

Yes, the rotting dead frog that actually STUCK to Dave's back was an all time low point here

I haven't thought about it too much, but as they roll in fox/deer etc poo also, I just presumed it was related to disguising their own scent in order to furnish them with some great hunting stealth mode - like an invisibility cloak, only not as pleasant
