Vinegar is acetic acid. It is made when yeast eat starch and produce alcohol which is then consumed by acetobacter bacteria to make acetic acid.
I have a hard time imagining a bread smelling strongly of vinegar but all the ingredients are there. The bread was fermented by yeast and did contain alcohol before it was baked. If the bread was a little underbaked and picked up the right bacteria it could be trace amounts of vinegar.
Other options are that the bread picked up a normal sourdough-esque bacteria and just smells sour without having actual acetic acid.
Either of these options would require the bread to be under baked or very moist.
Anything beyond this would be conjecture.
Maybe the bread was baked with vinegar in it?