2013.04.03 Leno fired because of Obama
There's a rumor that Leno was fired because he was too tough on Obama (and the Obama administration was pressuring NBC). While it is theoretically plausible, and he was fired despite having top ratings, and I'm sure his fair treatment of anyone on either side of the aisle likely irritated some snowflakes, there's no evidence offered or whistle-blowers to confirm it, so I'm not giving it much credibility at all.
One fact checking site "debunked" it -- with no evidence either. Other than his replacement (Jimmy Fallon) was tougher on Democrats for some short period of time. But that's a silly defense. Obama admin wouldn't know who they would Leno with, and number of jokes isn't the same thing as ones that might sting or annoy. (That's how cutting/true/good the jokes are). So that seems more like excuse making than a good refutation of the theory to me.
However, the lesser point that NBC did it to attract younger viewers (according to unreliable news source Washington Post), that has the air of plausibility. It might be stupid to give up your large older base for a smaller but younger one, but in advertising dollars? That might be a smart move. Older audiences tend to buy less and less as they age, and start going to bed earlier. So making a move "for the future", isn't quite as dumb as it sounds.