Zoo tycoon 2- How and why do visitors keep getting into the pens?

Ok, so its annoying enough how animals sometimes escape from their pens, but somehow, visitors keep getting in! I'll sometimes notice people(that aren't employees) standing inside the pens! One time a girl was standing inside the cheetah pen and started screaming and think that the cheetahs were chasing her. So everyone else started screaming and running around too and they all are thinking "i don't like this zoo". Why is this happening and how can i prevent it?