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.