Once it happened to me with a frozen package of cream it was in a multilayer tetrapack container and after only a few seconds, the aluminium foil inside started sparking and in a second a big flame erupted in the oven ehehhe, I really got scared for a moment!
Why would a grape do that? There is no metal in it, and probably 98% juice anyway.
Years ago we had a neighbor who put in a TV dinner in their microwave, the one with the metal tray. They didn't put it in the box as they were supposed to before cooking. It sparked, it arced and the woman left it in there to continue cooking as she thought "maybe that's the way it cooks" It burned black spots in a few areas of the SS interior that were permanent. I surprised she didn't blow the magnetron.