KG didn't touch Smith.
He absolutely did. KG had position under the rim, while Josh had his back to him also trying to establish position. He lifted his left leg to take a step back, but before he could plant it, KG planted his leg under it. So when Josh put his foot back down, KG's thigh had lifted his knee from underneath. This caused Smith's weight to come awkwardly on the leg, buckling the knee.
I have it on my DVR right now, and I've rewatched it 5 times as I typed this. KG obviously didn't do it on purpose. But it was a result of contact with his thigh/upper leg.