I'm willing to bet that the plane does prevent the pilot from activating reverse thrust when not on the ground.

But what if a thrust reverser self-activated, without being commanded? The previous safety mechanism wouldn't help because it's being bypassed.

So there is a secondary safety system that detects such situations. Something along the lines of "I think the trust reversers shouldn't be activated right now, yet they appear to be activated. The engine is clearly malfunctioning, lets shut it down"

It appears that this secondary safety system has been activated. There was probably a bug, or a sensor malfunction that triggered it.