The Patriots were in need of an offensive line coach after Dave DeGuglielmo's contract wasn't renewed. DeGuglielmo was the coach the Patriots hired to replace Scarnecchia, who retired after the 2013 season.
It's actually the third stint with New England for Scarnecchia, who turned 68 a few days ago. He also coached the team from 1982-88 and from 1991-2013.
Scarnecchia is highly regarded by both the league and Patriots' fans. The addition of Scarnecchia could provide the boost to the Patriots' offensive line that is badly needed after a poor 2015.