When a fighter falls splayed out where he isn't moving or knocked stiff and/or the body is showing signs of involuntary muscle spasms It signals brain trauma and or neurological issue and the fighter doesn't have control over his body. The ref stops it because even he did get up he wouldn't be allowed to continue as someone said above. While im a boxing and MMA fan.. I'd rather see a stoppage like that than the ones in MMA where a guy is ko'd and still receives a double tap of hammer fists before the ref can insert himself in between the fighters.