Same here. I find it hard to believe that someone went to a couple of their friends and said "I'm pissed off with my boss/coworkers/job/whatever, do you guys want to come shoot the place up with me?" From what they're reporting on the news this was well organized and targeted. They got in, did what they came to do, then got out. Very few mass shootings involve multiple shooters. When there are multiple shooters, all shooters usually (always?) were affiliated with their target and had similar grievances.
Given that it was in SoCal, my immediate thought is that it was gang or drug related.