In some few cases like (Foreman's) am not sure it's a fear of getting their "ass kicked" as such perhaps more a sense of self, ego, identity, self belief perhaps that gets messed with. Perhaps depends upon the self-image and public image too they have going into the fight too. Plus it's on such a public stage too. Probably depends on the manner of the defeat too. Wonder if it's more common in the fighters built up to be destroyer types.
Maybe losing their zero "frees" a boxer mind. Before that might it be harder to try something different i.e why change a winning formula? Might be talking our of my arse though
My absolute favourite is Stanley Ketchel when he lost to Billy Papke. He just disappeared, not even his management knew where he was, or pretended they didn't if they did. Went to the desert and when he remerged he seemed a different guy and knocked the absolute **** out of Papke in III.
You're not talking out of your proverbial at all. It can free a fighter up and also avert him from pretensions of invincibility that could, in their own right, lead to his ultimate downfall anyway and damage his psyche as it suddenly fell through a trap door - see first career George Foreman for one example.