They caught fire at the end of last season, and people expected that form to continue.
An actual look at the form table for the second half of last season suggests that Newcastle was crazy lucky. Third on points in the second half with +5 GD and twenty-five goals scored? That is a crazy lucky distribution of goals relative to concessions, considering that they won two-thirds of their games. They're on pace to post almost exactly the same for/against for the first half, but now they're drawing games rather than winning them.
We should have expected regression to the mean, which is exactly what we have observed.