Outside of the possibility of a large portion of people who voted for Hillary changing their minds, Im really just starting to think Sanders supporters are more likely to participate in national polls now. Shes leading the popular vote by a large margin and is about to win a host of states by large margins, this does not look like a tied race outside of national polling.
It's probably a mix of the two points that you brought up (Clinton supporters switching to Sanders and Sanders supporters more likely to participate in polls), plus Sanders supporters not voting or non-voters now supporting Sanders after their primary/caucus has passed.