I'm stumped.
I've had ceramic clay things spontaneously bust from some weak spot, but it's very rare. If it had, the fragments would probably be on all sides of the support instead of just one side.
An animal might have shoved the ball to one side sniffing around in there and overbalanced it, but a squirrel or bird wouldn't be pushing around a 5" stone ball.
Most vandals are opportunists. It's hard to imagine a stranger walking up to your front porch just to tip this over. Possibly a mailman or delivery man might do it by accident then bug out before he got blamed.
Have you pissed anyone nearby off lately?
Robert's Wolverine Theory is also attractive.