I have four ideas that I think make sense.
1. Christie is a political realist. Anyone not named "Marco Rubio" or "Ted Cruz" can see how this Republican primary race likely ends. And that's with Trump as the GOP nominee. (The math is close to conclusive.) Why not get on board now with Trump and perhaps reap the rewards (more on that below) rather than continue to fight a battle that looks increasingly unwinnable? Christie, like most successful politicians, is a pragmatist, not an idealist. And, for him, this was the most pragmatic move.
2. Christie wants to be in the vice presidential mix. This is the rapidly congealing conventional wisdom aimed at explaining Christie's endorsement. And, while I think it's not the only reason that Christie did what he did, it's absolutely a factor. Christie is term-limited out as governor of the Garden State in 2017 and clearly would like to keep up a profile on the national stage. By endorsing Trump right now -- the real estate mogul is on the verge of seizing a commanding lead on Super Tuesday -- Christie puts himself in position to be considered for the national ticket. (I wrote a piece about Trump's vice presidential list earlier this week; Christie didn't make the cut then but would now.) And, even if Christie doesn't wind up as the VP pick, he'd likely be in line for a Cabinet post -- attorney general? -- in a Trump administration.
3. Christie hates Marco Rubio and Ted Cruz. Throughout the campaign, you could see Christie's resentment toward Marco Rubio building. The New Jersey governor viewed the Florida senator as a central-casting candidate who always said the right things and looked the right way, but, when you scratched the surface, you found nothing there. Christie's famous/infamous takedown of Rubio in the New Hampshire debate just before that state's primary was a culmination of that anger.
As for Cruz, Christie views him as the epitome of everything that's wrong with politics -- a guy who made his name by obstructing things in Washington. Plus, Cruz's out-front social conservatism makes him a mismatch for Christie.
If you think there are only three candidates who have any chance at all of being the nominee and you actively don't like two of them, then you are left with Donald Trump.
4. Christie has a personal relationship with Trump. In announcing his endorsement, Christie noted that he has known Trump for a number of years and that the two men have worked together effectively in the past. Makes sense. As the governor of New Jersey, Christie likely had more interactions with Trump than anyone else who is running (or did run) for the Republican nomination. “Donald Trump is someone who when he makes a promise, he keeps it,” Christie said. "No one is going to get inside this guy's head. There is no better fighter than Donald Trump." That's standard political rhetoric but slightly more believable from Christie than from most politicians regarding Trump.