Misrani: What happened to the sibling?
Claire: She ate it.
Masrani: You created a monster!
Henry Wu: Monster is a relative term. To a canary, a cat is a monster. We're just used to being the cat.
Masrani: The key to a happy life is to accept you are never actually in control.
Owen: Time to take a stroll through the woods. 65,000,000 years ago.
Answer: Chances are either the gates were left open after the Pterosaur attack to allow visitors to escape quickly, or Hoskins had left them open earlier and forgot to close them to try and justify using the raptors as weapons.