[Blarg]

Since the BB is almost a guaranteed call with anything, plus blinds aren't catastrophic relative to button's stack size, and UTG and SB both have huge stacks in comparison that would probably happily call a steal attempt if they had reasonable cards, button would have to have a hand with some showdown value.

It might well be a hand that is vulnerable to the flop, though, since he didn't try to come in slow and entice people in. Something like TT, JJ or QQ or AK or AQ?

If the monster stack, UTG, is just playing every hand to try to hit something or steal by virtue of his intimidating stack size, button could have some confidence raising with less, especially if he has a tight image or hasn't done much stealing. Lower pairs or worse aces or even KQ or such, in that case? Reads could count for a lot here.

Sometimes I try to entice people in when I'm holding AA by not betting too hard, but sometimes I just plow all-in hoping they'll think it's probably a steal, so I can be sure of getting paid off even if the flop comes up not useful to the caller or a little intimidating to him. All-in being a bluff that I'm actually weak, when I'm not. Who knows, he could be doing that, too. Or just the type that loves to shove money in the pot not for deception but just to get paid off big straight out.

I am curious what he had.

As to UTG calling, he's getting 2375 for 1250 by calling; lots of people would call with any pair or with AKo here; it depends so much on the person. Some would call just for the chance to bust out someone on a steal. At 2 to 1 for your money, the money's so damn good that calling with some real crap wouldn't be all that bad. Even if he lost, he's still pretty much guaranteed ITM and is lodged solidly between 1st and 3rd. If he wins, his chip position has become very dominant and he's eliminated a threat.