.5/1, bad river call?

[Regis]

PF: I complete with A9s and raise w/ ATs. A9 doesn't even stretch to a straight. What makes it so strong that it deserves to be raised from the SB?

FLOP: I might bet out but am more likely to check and see what happens. I might like to avoid BB or MP raising and knocking out players and or I might want to take advantage of a BB bet to trap everyone for an extra bet.