The adrenaline's pumping. It's finish or be finished in the cage. The action in the Octagon can push combatants to the brink mentally, so if a fighter throws a few extra punches to close out a fight or holds onto a submission for a split second too long it's understandable.
Then there's cases like Paul Daley, Renato "Babalu" Sobral and Rousimar Palhares.
Daley and Babalu were dumped from the UFC for dirty tactics. Daley sucker punched Josh Koscheck just seconds after their bout ended while Babalu refused to let go off a rear naked choke he had locked onto David Heath. Palhares wasn't fired but he was suspended for refusing to let go of a leg lock for a few seconds against Alessio Sakara at UFC 111.
UFC 123 was close to having a similar situation unfold when Maiquel Falcao seemed unwilling to let go of a choke he had slapped on Gerald Harris. Two issues saved Falcao from being labeled a dirty fighter. There was a major clock snafu. The round was ended six seconds prematurely, and Harris came close to tapping on several occasions.
"He tapped out in three different occasions, hiding it. It was a new thing for me, it was the first time I got three wins on the same bout," Falcao told Tatame.com. "On the first round, he tapped out the first time when he was on the four points position, then he tapped out again in the end of the first round, when I got him on the rear naked choke with his body turned up, and on the third round he tapped out again on a rear naked choke."
That's a plausible explanation, but there is one troubling issue with Falcao. Just like Palhares, he has a history of ignoring officials in the cage at the end of fights. This 2007 fight in Brazil is pretty disturbing. There's no clock malfunction or confusion about the fact that the fight is over, yet Falcao keeps pounding away (0:17 mark) as the referee is trying to pull him off the battered fighter.