I think he’s losing his eyesight or something similar.
He’s fine in his stall - where he knows everything.
As my mare got older, I had weird reactions from her about things like shadows of flies (shadows on the ground) - flew backward one day and I thought she would flip over. Looking back, these were indications that she was losing her sight. She probably wasn’t able to reconcile the image and then panicked.
Your guy’s behavior sounds similar. Fight or flight is the first reaction.