Mother gorillas are fiercely protective of their babies, so this guy's mom wouldn't have let him out of her sight unless something had forced them apart. That's why rescuers believe that his mother was killed for the illegal bushmeat trade, a black-market industry that kills thousands of primates every year in Africa, and which all too often leaves baby animals without parents.
Some of these animals end up being trafficked and sold as exotic pets. When they get too big to handle, owners don't want them anymore, but the babies have forgotten too much of their own nature to fend for themselves in the wild.
Luckily, in this baby gorilla's case, he was saved before he could experience any more trauma.