Electric Forced Air Furnace

Why would a strong flow of air be coming out of the cool air return grill in an electric forced air furnace? All heat registers function well. Thanks.

Detached duct and high winds?

Detached/ closed off return duct with cool air from rim joists/floor cavities going to another operating return register. This area of the house is under negative pressure in relation to areas with operating supply ducts/grilles.