I have a 2006 Ford Explorer the back hatch window will pop open when cold out and driving down the road
Your lock latch could be freezing and not closing all the way, the hatch lift supports (aka struts/pistons) are keeping it closed at slower speeds, but can't as you pick up speed. Try thoroughly cleaning the locking mechanism with de-icer and/or graphite spray. Check also that the lock lines up with the striker on the body to ensure that it can close properly. Hatches tend to accumulate a lot of moisture so it's easy for things to freeze (and rot) in there.
Search Maintenance & Repair Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Maintenance & Repair Experts