The simple answer is when visibility is lost if you hit someone, you were obviously not slowing down enough for the conditions. But, that's the simple answer. There's also the factor of cars slowing down too much to expect everyone else to slow down that much.
Does the fact that it's only practice play a factor? Maybe. Maybe not. We all slow down in situations like that based on what we feel is safe and under the conditions of the session (practice, qualifying, race). But again, one driver's "I slowed down enough" is another driver's "he slowed down too much."
Compounding the equation is from the in-car video, they both look wrong. The trailing car's viewpoint makes it seem like the car in front slowed down way too much. The car that got hit's rear view camera makes it look like the sedan did not come close to slowing down enough. Again, it's all relative.
If I had to make a call, it would be the sedan's fault, simply because he didn't slow down under the conditions where he could not possibly have known what was in front of him. And that's the safest thing to do. Sadly, this reminds me of the horrible crash at Summit Point in an SCCA club race, way back in the 1980s I believe, during a race which killed both drivers.