According to ESPN:
A teenage girl ran at Lochte, who tried to catch her, but the two fell over in the process, his publicist said. Lochte hit his knee on a curb, while the girl was not hurt Sunday.
Publicist Janice Lee says in a statement issued Tuesday that Lochte's medical team expects him “to make a full and speedy recovery.”
If this were anyone else I'd be thinking what a fucking moron but, oddly, this seems like a fitting way for him to go down. Even before I read this list of odd ways Lochte injures himself. Also, how big was this girl?