Swelling of lithium cells in cargo bay
A -4 psi delta pressure on a cell could cause the casing (metal or plastic) to bulge outward and deform the internal current collectors and create an internal short circuit. A shorted cell could vent or burst and produce noxious fumes and/or fire depending upon the chemistry and size, energy content, etc.
Now i have to question whether MH370 did indeed carry lithium cells in the cargo bay, or if that is just a rumor?