I think it's due to his opening range from the SB is pretty big. If you don't think he is going to fold any of his range then 3betting is fantastic as he calls with all of that range which AJ beats.
I also don't think that he will 4bet unless he has a very big hand, in which case you can fold quite happily knowing you would have lost more if an Ace comes up on the flop as you can't really fold a lot of the time.
It means you can cbet the flop and he'll be more likely to fold than he would be* by you calling and if you hit the flop you can just bet for value every time as he'll be calling with worse hands so it's profitable.
* Example
you 3bet, he calls. Same flop comes down he checks to you, you can cbet and you have decent fold equity and if he calls you can be fairly sure he has a piece of the flop which means that your Ace is likely to be pretty good on the turn.
edit - Also from tomorrow I shall be playing 5nl, so watch your back.
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