There were 18 chimpanzees vs. 1 silverback. While the silverback was getting harassed by 10 chimpanzees, the gorilla infant could be targetted by the other chimpanzees.
Moreover, the silverback was troubled by the presence of human observers and fled 3 minutes later, whereas the chimpanzees were habituated to the human presence.
Quotes below:
> In both events, the chimpanzees considerably outnumbered the gorillas. [...] At 17:13, the silverback charged an adolescent female chimpanzee, Gia, knocking her into the air. At 17:15, a group of approximately nine male chimpanzees (adults and adolescents), and at least one adult female chimpanzee surrounded the silverback, and repeatedly jumped down on and hit him whilst screaming and barking. [Meanwhile, the gorilla infant could be kidnapped by the other chimpanzees.] At 17:22, one adult male chimpanzee, Littlegrey, was observed sitting on the ground holding a gorilla infant in front of him.
> During the first encounter the silverback was present for 14 min [out of a total of 52 min], whilst in the second encounter the silverback was present for 10 min [out of a total of 74 min]. [...] At 12:33, the silverback discovered the human observers on the ground in a distance of approximately 30 m to the base of the tree and started barking. [...] At 12:36, the silverback rapidly climbed down the tree and fled. The chimpanzees continued barking but did not follow him. [...] We cannot rule out that the presence of human observers, in both events, may have had an effect on the unhabituated silverback’s departure and may have tilted the imbalance of power in favour of the habituated chimpanzees.