Also, keep in mind, these are gorillas. A very different way of thinking than humans have. i realize that they are personified for the movie, but in reality, it's every gorilla for itself, for the most part. A mother would protect her young, for the sake of carrying on her genes, mostly, but once the leopard already has it, there's really nothing that can be done. It would be better to keep yourself alive.
Also, as a point of interest, as far as I know, the father would have no idea which of the offspring are his. I don't know that theat applies to gorillas, necessarily, but it applies to chimps, so let's assume. The male gorillas will mate with any female who is in estrus at the time. Several males might mate with the same female in a close amount of time. That serves to protect the offspring. if none of the males know which one is carrying their all important genetics, they will be less likely to kill the offspring. Which is something else that is seen amongst primates. There isn't really a mother and father unit amongst primate species (still assuming from the chimps) There is a mother, and a kind of community pool of fathers.
In a gorilla pride, there is only one mature male. All the offspring are his, it's a harem system. Not how it's done in the movie, of course, though.