When I first heard about this, I assumed it was e result of a Sikh inter-factional dispute. Those are not exactly uncommon. Then I read that the shooter was white, which means it was probably racially-motivated. My initial suspicion is that, if this is the case, the person just assumed that Sikhs were Muslims. It is cold comfort to think that he may have killed Sikhs by mistake when he was actually gunning for Muslims. However, since discrimination and hatred towards Muslims is much more acceptable than against any other group, this is a real possibility.
However, it is possible this guy actually had a beef with Sikhs. We will need to wait and see.