Baboons are prolific breeders - it's probably just been bred up over the years. There are other places with similarly huge groups (Knowsley, Cologne, Emmen?).
The initial troop of about 60+ baboons were sourced from 2 zoos if i'm not mistaken, one in the Netherlands and one in Sri Lanka I think. The two separate groups were mixed into the exhibit and from there on they couldn't stop breeding. There are about 90 baboons now.
