Why might food go bad in an oxygen-free environment?
First of all, you assumed that this tortilla went bad after 18 month, not that austranaughts just decided to eat it?

More importantly, there is thing called anaerobs. Just like your muscle don't use oxygen in times of acute stress (force requirement), so there are organisms that don't care much about oxygen. As it happens in muscle, it happens in yeast during fermentation when sugars are turned into alcohols and released energy is used to propel molecular machinery. So, if there are sugar molecules on tortilla, there are still microbes that willing to prosper on it.