If it were me, I would look at the following in this order:
1. Key fob being pressed accidentally (as eaglerockdude pointed out) - similar to butt phone call but this time you're pressing the rear window. Try keeping the fob not in your pants but a shirt pocket or somewhere you can't press it unintentionally
2. Key fob could be defective - circuit could have intermittent short. If you have 2 fobs, just use one and remove the battery on the second to isolate if any of them are causing the issue
3. The rear window switch on the dashboard (to the left of steering wheel) - could have loose wire or shorting for a little bit which opens the window. It easy to remove that one and check it with a tester.
4. The final one which is the worst but most unlikely is there is an intermittent short in the electrical of the rear window motor causing it to open a little bit erratically. This would be the last thing I would check if 1, 2 and 3 are all good.