I also would normally expect that a person would fold to the preflop raise with the 79 o. Then real hard to tell why he did not... he was a big stack in BB, figuring you want to steal... or some other reason...
But the big loss and big problem came with his shove. At that point as was said above - anything was beating your hand - as in any 2 pairs or 3 cards...your one pair of A's look real weak against a shove.
Very hard to fold - I know - after you invested over 1K chips and when you have the nagging feeling that he is bluffing
... but I found out the hard way that people do not really bluff as often as one would think.
Take this with a grain of salt - I am not a great player just my opinion.