Party 50 NL hand with AA

[Byrn]

Party 50 NL, I am new to the table and don't really know any of the players well.

I get AA UTG+1. UTG limps ($1), I raise to $4. All fold to BB who calls, UTG folds. BB is unknown to me, in the 5 hands I have been at the table, he seems to be in almost every hand, so he seems to be playing almost any 2 pre flop.

Flop Ks5h4h. I don't have Ah. BB goes all in for $39. I haven't seen him make a move like this in the time I have been at the table. I think he has been pretty tight post flop actually.

The other thing I know about the BB is that as I arrived at the table another player was berating the BB for donating money to him. Well, thanking him in a mean spirited way would be an accurate description. So maybe he made a move like this prior without much.

Would he make this strong move with any K? Would he need 2 pair? Is he tilting and/or trying to bully me?

My thought process went like this: Most of the time when I see this play on Party it is not a bluff, though I of course have seen this play as a bluff. It usually means a hand they really like (2 pair, set). I have only $4 invested, and I don't think I have the best hand anymore. I can get away from this and be fine with it. I would much rather be betting all in than calling right here.

So, I folded.

Thoughts? Flames? I'm a total Wuss?