What would cause a ridge vent to pull in chimney smoke?

Here is some information about ridge vents suddenly becoming an intake vent.

An exhaust vent can become an intake vent if it's starving for proper intake air at the soffit/eave/overhang/roof's edge. Check the linear footage of ridge vent installed, determine the total Net Free Area it is producing, and then compare that NFA to the amount of intake. The two NFA values should match. If not, it's possible the exhaust vent can ingest. Otherwise, Air Vent has not heard of a ridge vent pulling in chimney smoke.