Look at the parameters area of wmt. Any parameter google has ever used will be listed there. If it's something you never use yourself, tell them to ignore the parameter.
Last time I looked, I had three listed-- and I don't use parameters at all, ever. Matter of fact it was a nasty shock because I was just checking the area so I could tell someone what wmt tab it's under. Ouch!
Once google has met a parameter and found a valid page at the other end, they will probably keep trying the parameter forever, and may try others for the heck of it. So you have to tell them to cut it out. It's the same idea as periodically requesting garbage URLs.