Apparently, it's coming to light that Clarkson might have actually set up his punching the dude with the guy he hit ahead of time as a way of getting out of his contract. He was apparently trying to get sacked after he found out some of the top-level execs were comparing him unfavorably to Saville (that child molester ex-star) and had excessively enough.
Also, there are reports that the BBC tried to hire him back on after some of the more recent backlash to their new casting choices and the announcements of the new Not! Gear. Apparently, these negotiations fell through when Clarkson caught somebody on the BBC team once again comparing him to Saville.
If any of it is true, it basically explains everything, from why he hit the guy to why he reported himself and why the guy got away with only a split lip. It'd basically be a big Xanatos gambit on his part to drum up hype for himself and force a favorable outcome.
If it's true, since I'm working on a tablet at the moment and I'm having trouble hunting the sources for this down.