Sanders likely dropped because his victory is NH was not that impressive. He received over 60% of the vote in 2016. While no one expected him to do that in such a fractured field, 26% and winning by less than 2% was not impressive at all.
The results match the polls though. The only difference is Warren and Biden both lost late support to Klobuchar, which is good for Sanders.
Basically, if the polls on February 10 said Sanders was going to get 26%, and on February 11 he got 26%, why would he not already have been at his current odds on February 10?
Sanders was favored by more than 1.5% in the polls.
I think the bigger picture is that the "moderate" candidates did much better than the "progressive" wing. The majority of Democratic Primary voters are not sold on Sanders' "free everything for everyone" platform.