My old '01 F350 used to have a "self-locking tailgate."
When you went down a bumpy road, somehow the lock core would vibrate itself into the locked position.

I thought I was experiencing early on-set dementia because I just didn't remember locking the gate, and yet it was locked and I was the only one with the key!

Was finally able to reproduce it by driving in a rough section at the back of my property. Loaded the bed, did not lock the tailgate, drove across a couple of ruts and some gopher holes... got out 30 seconds later to unload and the gate was locked. Did it every time.

They wanted some ridiculous price to change the lock and re-core it to match the ignition key, so I just lived with it. I was actually more relieved that I wasn't losing my mind, and never had to worry about having my tailgate swiped.