Well... It is sucks!!

Here's a little story, hopefully has nothing to do with the OP problem...

Years ago when parts cars were plentiful and cheap, I got a 74 Duster. It had been hit in the rear sometime in the late 70's. It was fixed, they put a tailight panel in it, painted it, etc. Kid bought it, took it apart, started to sand it down, left it outside, blah, blah, you know how it goes..

I'm taking it apart with a buddy. I unlock the rear window gasket. Didn't touch anything else. Turn our backs for a second, and there is a strange hissing sound. Suddenly, BOOM! The rear window does what yours did. Come to find out, one side frame rail was bent, right at the peak of the curve over the rear axle. Like a quart milk carton if ya gave it a small crease on one side. The sides were slightly bulged out. It was either never caught or never fixed right. Unlocking the seam in the rear gasket must have allowed the rear window frame to shift enough to put some force on the glass.

Measure your window opening for square.