The same thing happened to my 1994 306XT a couple of years ago.

I came back from work and saw a pile of glass on the road next to the car (left hand rear door window).

The glass was all outside the car, very little inside, indicating a force outwards.

A lady nearby told me she heard a loud bang at midday and came out to investigate.

I had closed the through car ventilation system that very morning. I assumed that the interior of the car had heated up in the sun and the expanded air had nowhere to go.

It was a new learning experience for me to replace the glass, but I got it done.