I can see several:
* accident (no motivation) -- something happens (something fell, perhaps) that caused a mark.
* set-up -- in other words, to set blame on someone (whether an Egyptologist or whomever, setting up someone as the "fall guy."
* suspicion -- set up a claim that the cartouche is fake
* altering evidence -- set up a claim that the cartouche has been tested and is of a certain age (since they can assume that people aren't going to willy-nilly scrape up paint to test.)
I don't think we can determine the means (could have been any scraper) and you've covered opportunity.