The REAL answer is USUALLY because they have had contact with another another animal like for example a human and they got tier scent so the mother believes it's not theirs anymore or that they will cause harm.
remember most animals do not experience love attachments like humans do. they care for young up to a certain age because it's in their genetic coding to do so. only a few higher order animals like primates and , yes, elephants, have shown something akin to true love attachments. in most species as soon as a babe is old enough to leave it's mother it becomes just another animal and can even be mated to it's own siblings or parents
Well my hamster ate her's once and mom said that it was because she was afraid that us humans would harm them or take them but I was young so it could be false I usually don't trust what my mom said when I was like 5.