The trio were chasing lone leader Philippe Gilbert (Quick-Step Floors) with about 17km to go when Sagan manoeuvred toward a sliver of smooth surface next to the cobbles and appeared to catch his handlebar on the barriers. All three riders hit the deck on the Oude Kwaremont.
Van Avermaet quickly remounted and was back in the race, while both Naesen and Sagan were slowed considerably more. Sagan waited for a new bike, but the impetus for the chase, and his chances for a second win, were clearly gone.
"I think Peter Sagan was close to the barrier or the people and maybe he hit somebody on the side of the road and I was just behind so I couldn't avoid the crash," said van Avermaet, who went on to finish second from a group of three that came in 20 seconds behind Gilbert.
"That was the moment when maybe we could have still done something with Peter and Olivier Naesen," Van Avermaet sad of moments before the crash. "We could have worked together, but then you stop, you crash, and you lose another 30 seconds and then the race is over. If you see how close we still came at the finish I think the race was not over until that point."