re-raising with 99 is suicide. if youre right (ie, opp has big cards), you have sent the action back around. if they come over the top with their AKs, you now have to fold the hand. even if they flat call, you are less than even-money since they can easily CLAIM any overcard (1/3 of the deck). however, most times you will be on two outs to begin with and have now swelled the pot to the point where it cannot be profitable in the long run even if you do manage to set up.
HOw many players at 18/9 have you seen 3bet AK preflop? or for that matter push over a reraise?
Im not debating that 99 is a little risky, or that AK calling here is bad. However id ask if 18/9's really push with AK? or 3bet for that matter?