What would cause maple syrup bottles to bulge?
Even though the risk with canning/bottling maple syrup is supposed to be virtually nonexistent, it sounds like yours was somehow contaminated. I definitely would not use them as "bulging" cans or bottles are a prominent sign of botulism spores.

See the CDC's Home Canning and Botulism:


  Home-canned food might be contaminated if:
  
  
  The container is leaking, bulging, or swollen
  The container looks damaged, cracked, or abnormal
  The container spurts liquid or foam when opened
  The food is discolored, moldy, or smells bad
  


That said, it's not necessarily botulism, that's just the worst-case scenario - but it definitely sounds like some sort of bacterial contamination which is usually due to either improper canning or improper storage. What you're seeing is the result of a pressure build-up, and the pressure has to be created by something, and that "something" is usually bacteria eating the food inside and producing gas.