Despite this looking like a very straight forward incident, it’s possible multiple factors were at stake.
Thinking that the driver of the white van didn’t check their mirrors is by far the simplest explanation, which is often the correct one, as per Occam’s razor.
But what if they did check their mirrors and actually thought they had time/room to depart?
This is where we need to consider the possibility of that SUV going way over the speed limit. The sheer force with which it rolls and the fact that it can be seen flying past two dashed lines in what amounts to 1 second, indicate that it could have been carrying a lot of speed before the impact occurred.
Somebody with the username parhelion12 claimed on reddit that they were actually driving that SUV and that they were doing 80 mph (128 km/h). Whether that’s true or not, we don’t know, but that number would indeed put you over the speed limit on the Pacific Coast Highway in California.
What we know for certain is that that white cargo van could have waited an extra couple of seconds before performing what looked like a 180-degree turn. Toward the end of the clip, the slow-motion zoom part reveals that the van was previously parked by the side of the road, which could have influenced visibility, though not by much.