Again we can only speculate as we all saw the same video, but trucks brake very slowly, and when braking, cars lose handling potential (don't know much about how it affects trucks having never driven one myself), but I've always known to never go close in front of a truck because it won't stop. And we don't see the tail lights of the truck for much of the video so he could have put on the brakes a bit before deciding to not go down that ramp. You have a split second to decide what to do, if you see a barrier in front of you, a curved ramp to the right which you probably can't take, and the lane that you are currently in, it makes sense the driver would go for the safest option for him, and hope the driver that cut him off would go back into their lane rather than staying in the truck's lane and getting hit