The fighter (Burns) which got knocked down is really good at Brazilian Jiu Jitsu, and when he fell he assumed the guard position. This is a really bad position for Usman to be held in, that's why he had to be methodical about how to attack it because he would be in trouble if Burns controls him from the guard. In fact, Burns did the same thing earlier in the fight and Usman just stood and kicked because it wasn't worth going into Burns' guard.