Dust, bad rubber band, screwed up lube on the stepper rails. All sorts of stuff can go wrong but is relatively easy to try and fix if you open the unit up.
Obligatory: not a safe class laser device, do not operate with the case off or with grease (fingerprints) on any of the optics. And don't do you "first test" of your drive by burning a disc, that dramatically increases the power levels (to most of a watt IIRC).