Sometimes, if a mouse dies naturally, the tankmate mice will eat it to dispose of the body. (in nature, this would presumably prevent a rotting mouse corpse from smelling and thereby attracting predators to the mouse den.) I have heard this happen when a female gives birth to dead young; that's why you sometimes see the gruesome sight of a female eating her young; they were probably defective and not able to survive anyway.
Maybe there was just some sort of disease in your situation though.