I happened to be directly behind the rider when this happened. It really messed with my head seeing this happen about 15 feet in front of me and really made me reflect for the rest of the day.
I had been riding behind him for about 5 miles and we were doing the usual commute/traffic jam type of ride in the express lane (sometimes filtering between cars, sometimes riding normally in the lane). Everything seemed fine and I didn't see any concerns with his riding. Eventually he wanted to exit the highway and signaled to the right. There was a gap to the right of us and he successfully moved one lane over. He then tried to move another lane over to the right. There was a gap in this second lane but unfortunately there was a semi truck quickly closing that gap. My guess is that he didn't notice this semi at first. Once he started moving into the second lane, he noticed the semi and then had to move back into the first lane. During that short period of time, the traffic had come to a stop in that first lane. He ended up rear ending an SUV at a pretty good speed. I don't remember the speed we were going at that time, but this was not a slow impact.
The amount of parts immediately sent flying all over the highway was surreal. The SUV was totaled, the rider went flying a very very far distance, and the bike went under the semi in that second lane over.
We were just two guys on our way to work, we were both having a normal day, and now he's no longer with us. That really messed with me for the rest of the day yesterday. I've never seen a crash like that happen directly in front of me.