I think Pop wanted Leonard on Westbrook because he anticipated a Russ/KD PnR. The Spurs would switch that, which would put Kawhi on Durant and Green on Westbrook. In the previous games, Parker and Leonard got confused over how to defend the 1/3 PnR, and Westbrook kept getting into the paint because the two Spurs kept going to Durant. However, Brooks went away from the 1/3 PnR in the last two games, because he didn't want the Spurs to switch it. He thought that Westbrook had the advantage over Kawhi and Durant had it over Green.
He ended up being dead wrong.