Electricity follows the path that has the lowest resistance. When a ground becomes incapable of handling the amount of electricity/load being drawn it will take the next path of least resistance through a component that was neither designed or intended to carry the electrical demand. This can cause the component (s) that is the "new" ground to either react in very strange ways and/or be damaged.

As an example, cars that had clutch cables with bad engine grounds would start with odd electrical symptoms and eventually the clutch couldn't be disengaged. The clutch cable was the next path of least resistance but was capable of continually carrying the load/current so the inner cable would weld itself to the outer casing.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer