Now I have the answer as Peter Sagan has appealed the disqualification! I assume a final answer will be rendered soon as Sagan must start tomorrow's stage or he's out.
I had seen the video of the crash on my phone and also on the tv/computer and thought that it is a BS disqualification just sprinting and getting position. But then I saw the overhead view from the chopper:
Check the last 10 seconds. He probably should be DQ'd.