Physical Danger
Clowns are all about slapstick, and when is the last time slaps or sticks put anyone in good health? Any time you're trying to make it look like you're getting hurt, there's always the risk that, you know, you'll actually get hurt. A pie in the face has the force of someone's palm behind it, falling over can land you on rocks or other unpleasant terrain, and don't even get us started on messing around with circus animals.
That said, unlike most dangerous work, almost everything that endangers the safety of a clown is self-inflicted. So before you "pretend" to wallop yourself in the face with a cast iron skillet in exchange for eight dollars an hour and the chuckle of a six-year-old, put some time into training and practice.