what would cause domestic cats to eat each other? one cat died and
some other cats ate it.

What most people seem to forget is that cats are domesticated. This
does not mean that they lack certain instincts. One of the insticts
(epsecially with cats) is that a free meal is still a meal and as
such, increases chances for survival. Another possibility that is more
likely, unless you specifically saw cats eating the other, is that it
might have been some other wild animal.

Or, perhaps, the cats were simply hungry. 

Kitten-ka-bob, anyone?