I watched it from the start (with a big pile of ironing and my DH out racing). PS was protected from the start. He was hard to see but was consistently near the front. Daniel Oss was playing a blinder as a super domestique for Sagan.
If you watch the last few km before Sagan goes on the break you can see him following wheels but not looking best pleased. He had criticised other riders for not working last week in Flanders and I was surprised to see him tucked in to the group. I reckon he got fed up and went for it.
The break hadn't been mopped up ahead of him and I think the peloton just thought, "let him go, we'll reel him in again". When you see who was in that group there were plenty of big names who were more than capable of getting him back. Dillier is one to watch for the future, no-one gets to be Swiss champion without being handy.