Jacky Durand on French Eurosport did a frame by frame analysis, as Merlier got our of the saddle to sprint hus bike obviously swung from side to side and clipped Ewen so one of those things. He was, however, very carefully not suggesting that Sagan had nudged Ewen into a position where there was no space
Merlier kept straight, but getting out of the saddle always moves the rear wheel back a bit.
That would lead to Ewen's front wheel overlapping Merlier's rear wheel, and if there isn't wobble room, Ewen goes down. Every cyclist ought to know not to overlap wheels unless there's a good amount of lateral space.
Yes, Sagan had nudged Ewen over so there wasn't space to try to go round Merlier on either side - barriers to the right, Sagan on the left. It's called tactics, and meant that Ewen would have to wait until either Merlier or Sagan jumped before he could go. As it was, Ewen was a bit too keen, and overlapped wheels.