This would happen on my CC Plus on occasion and is a total mystery to me. I replaced the belts and was good for about a month, then it did it again. When it does this, it seems to stretch the belt ever so slightly, so it may reoccur with more frequency. Every time it does this, it damages the styrofoam on the side. I had to replace that once as well. I was never able to figure out why this happens and never actually witnessed it happening either. And besides the black marks it leaves all over my plaster pool, its a decent robot despite this.
BTW, I just sent mine in for warranty repair, for the same reason. Track came off for the last time, and never acted right afterwards. Motor malfunction. Bought it back in June 2019, so clearly longevity with these things is questionable. I purchased a Doheny's Discovery in the meantime, and will have the Nautilus as a backup.