Door trim on 1995 Volvo 940 2.3 LPT

I have just bought an example of the above car. On the three others I looked at the interior door covering (vinyl?) was buckled up in several places on the horizontal section running along the base of the windows (see

for an example).The car I bought does not have this problem except for a ~0.5" section atthe rear of one of the rear windows. What is the cause of this and is there anything I can do to stop it spreading or occurring elsewhere?

And, does anyone have any suggestions for tidying up or disguising the resultant mess when the problem does occur?

Thanks, Matthew