Comparing Kaveinga and Manti choosing USC over BYU is quite different for two reasons.
First, Kaveinga is from Cali and all his friends, coaches, and neighbors had been putting pressure on him to go to USC, plus I believe he was a huge USC fan if I remember correctly. USC was the hometown school. Manti is from Hawaii and it seems that he has no desire to go there although I'm sure there's a lot of locals who tell him they want to see him stay home. Fact is, that's not going to happen. I would bet there are more people close to Manti pulling for him to go to BYU than USC which probably wasn't the case with Kaveinga.
Second, Manti will have 2 or 3 relatives already on BYU's team, not to mention all of his relatives that live here and help out with the team. Kaveinga didn't have that.