My guess is that Kiprop was doping for a while, stopped, and then went back to it. Kiprop was one of several Kenyans managed by Frederico Rosa. In the first half of 2016, Rosa got busted for doping some of his athletes. Then a funny thing happened: even those of his athletes who had been stellar performers suddenly couldn't do much. (Take a look at Kiprop's performances from just before the Olympics and continuing after.) He must have decided that the risk was worth it.
There are now getting to be way too many Kenyan dopers, which will ultimately damage a reputation all Kenyans have for long been proud of.