Follow this: The chimps escape. They are doing whatever chimps do when they escape.
The couple arrives.
The chimps see them and remember them. They attack.
I've seen dogs do this. What problem do you have with this?
Animals don't have human intelligence but they do have social structure and they simply look at humans as other of what they are. That's why they're dangerous.
Chimps are smarter than greman sheperds and I've seen german sheperds to things that require as much recognition as this attack did. I've seen a dog launch an attack that required MORE functioning brain circurits than this particular attack did.