Looks to me like his right foot was aligned slightly to the left when he move forward with that leg to jump. putting some sideways pressure on the knee to jump. Like thoroughbred racing horses, these guys are so freaky physical on their size, build and musculature, that it doesn't take a lot of misalignment in a move that they are constantly using to cause an injury. Especially when they are so young and still growing into their mature size. Building the muscles that protect the knee is greatly needed because the pressures put on the knees in this sport are extreme.
Just a slight misstep and we lose his talent for a time. The kid deserves a few prayers from Spurs fans for a fast and full recovery and rehab.