I think it is 50 percent psychological and 50 mental as one boxing joker might say (Tex Cobb comes to mind both as a quote maker and an example of a fighter who suddenly caves in to a nobody after taking bombs from Shavers, Holmes and Norton).
Psychologically the fighter probably just has no heart in taking so much punishment any more. Mentally, as in the brain's ability to absorb the punishment of bouncing around inside the skull, just isn't there any more either. Like any other part of the body, the more damage there is to it, the less chance of resistance to further pain there is.
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