There's a coverage in the Homeowners policy called Medical Payments, usually $1,000 or $2,000, that the insurer will pay out without regard to fault. The idea is to offer someone who's had a minor injury on your property a small amount to make them whole, without the hassle and expense of attorneys. They may want to recover under that.
We had a pool in our last house and the stubborn 4-year old son of a friend jumped off the side of the diving board after being told repeatedly not to, and ended up needing a few staples to close a gash in the back of his head. I offered to file a Med Pay claim if they had to pay cash out of pocket (e.g. a steep deductible) and they said it wasn't necessary.