Because he's just so very, very tired. Let's face it; by and large, ladies are far better at arguing than dudes. They've generally got a higher EQ, they usually care more about whatever it is you're arguing about, and they've got the endurance of a yak.
There's this thing called "learned helplessness," which is basically when someone gets smacked around or shot at or chained in a dark room for so long that they just give up and become resigned to this being their normal state of affairs. That's why people who have been kidnapped and held for decades will sometimes have an opportunity to free themselves and just say "eh, whatever" and go back to their corner.
THAT'S WHAT YOUR DOING TO YOUR BOYFRIEND.
No, not really. He's just sick of arguing and probably knows he's not going to win this one. Most guys are also pretty good at detaching and going into robot mode, so if you've hurt us deeply or are annoying us to the point that we fear we will soon start hitting you and just never, never stop, often "shutting down" seems like the most painless option for everyone involved.
Next time this happens, just give the guy some space. Whatever the issue is, he'll be fine to talk about it later...right now, he just needs to go be alone and dead inside for a little while so his emotions can get back in check and his head can clear. Anything you say before then is just going to build up the resentment that's even now gnawing on his insides like a rabid wolverine.