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Could be high humidity on either side of the drywall. Or and most likely, the drywall was too thin for the joist framing. I've never had one and never ran across anyone who had one, but according to regulators cars are bursting into flames everywhere.
They require 5/8-inch drywall in attached garages with joists spaced 16-inches on-center. You may have either or both 1/2" drywall or joist spaced farther apart than 16-inches. Thus, the drywall can't support itself or even any insulation on top of it and sags.