More than likely you've "nicked" something like a small rock with one or two teeth.  This will make one side of the chain "out cut" the other side because it effectively has one or more aggressive teeth vs. the other side (two sides to the chain, usually the cutters on the right will be affected since this is the side next to the ground when 'felling').

The more teeth affected, the worse the side cutting will be.

edit: The other thing to avoid is wire, it seems like it is magnetically charged and drawn to chainsaws, and can definitely affect one side of cutters more than the other and it doesn't have a favorite side of the chain either.