I think that people are answering this too quickly without giving it enough thought.

Suppose I am in the SB and Phil Ivey is the BB. Everyone folds to me.
Ok, what am I thinking ?
Well, firstly I’m sh!t scared of having to get involved in a hand with Phil Ivey. If we get to see a flop, then he’s definitely going to outplay me, and I’m going to lose chips, so the best EV decision for me is to fold.
Even if I have KK or QQ, I’m going to fold, cos if I raise and he calls me, I’m in deep trouble after that because he’s got position, and well, because he’s Phil Ivey.

Then I look at my hand – oh no, AA.
Disaster !
What do I do ?
I know, I’ll go all in. That way I cleverly negate his positional advantage. He can’t outplay me because I don’t have to think anymore.
So, that’s what I do. All in.

Ok, now look at it from Phil Ivey’s position. He has AA and the SB has gone all-in.
Because he’s Phil Ivey, he reads the SB like an open book, and immediately draws the conclusion that the SB has AA too.
So if Phil calls all-in, then 95.65% of the time, the pot is split.
But 2.17% of the time Phil wins, and 2.17% of the time Phil loses.
So Phil says to himself,
“why take the chance of doubling up when there’s an equal chance of me being knocked out ?
I’ll get lots of better opportunities later.
So, I fold.”

Result : Instant fold.