Well, for example, it would help if the failure was during the night- no lab workers around to fix things, except for, realistically, people with nightshifts. Perhaps an electrical shortage unlocked cages? Set fire to the building? Perhaps one raging creature managed to break free and wreaked havoc throughout the lab complex? It's up to you.
Afterwards in the confusion, the creatures would likely disperse, but end up encountering others, and possibly banding together because of the realization that they're all in a new environment and need to survive together. Something like that, I'd think.
