I've been observing more and more of this behavior recently...I should mention lane sharing is legal in California.
I'm not one of the fastest guys on the road, ever, but certainly not during my rush hour freeway commute. I'll split lanes here and there, usually at a comfortable/cautious pace, but often I'm fully in the lane, not sharing it with a cage. If I see a bike coming up behind me when traffic is moving slow, I'll move to the left part of the lane, very obviously, and wave the rider by. I'm happy to let him/her by...no skin off my back...
What's been happening more frequently is while I'm on the freeway, traveling with the flow of traffic, bikes will come up behind me quickly (i.e., faster than the flow of traffic) and split lanes, at times between me and a cage, and at other times they will get up close to my arse, dart out just barely into the next lane, and then off to weave in and out of traffic.
Today, I had two bikes behind me, one raced passed me in the above manner. This distracted me and I drifted a little more towards the right portion of the lane than I expected, I move back to the center and see the other bike in my left mirror as if he was getting ready to pass me in my lane on the left. I couldn't believe it! I was in the fast lane going with the flow and this would have been between me and a guard rail (no shoulder). I did a little weave in the center portion of my lane to protect my lane, and he eventually changed lanes and passed on the right weaving in and out of traffic. Damn, it scared the hell out of me and pissed me the f off! It's totally disrespectful to another rider, and even more than that it was dangerous and stupid and their intentional behavior puts my life at risk, and that pisses me off.