Nothing wrong with your call, short stacks get desperate and you dont know if they are trying to buy the blinds with a strong hand or if they just hope you think they have a strong hand when they are really weak. He could have had nothing more than A2 or A3... when your short stacked and havent seen any good cards, any ace may look good. Fact is, his 55 isnt a "monster hand" and you had one of the 5's... so you had a lot more chances to catch an ace than he did a 5.