Not seen this on mine, but it could suggest that it's loose on its runner?
Which is the polar opposite of mine which has become incredibly stiff over the last few weeks and will barely budge either by human or motorised means.. a bit of WD has made it at least prise openable, but it's only a matter of time before it tears itself to pieces. Oh and my other door snapped its cable, so is a manual door now and really loose on its runners....
I guess with some of these buses around 20 years old now, these kind of mechanisms can wear out.
Sorry not able to offer any useful suggestions.