There was some sort of emergency,or the plane wouldn't have turned around and gone back once inflight.
@sneakypete Sometime later yesterday, Pence's secretary said he was called to the White House before his plane left the ground. She also said it was not an emergency. I think that is bovine scatology as the White House isn't saying why he was called to the White House and cancelled his trip to New Hampshire and all the people were gathered in New Hampshire - I saw a short film of all the New Hampshire people leaving the place where they were gathered, waiting for Pence.
For some reason, Pence cancelled his trip in the time period where the people were already gathered waiting for him. While Iowa holds the nation’s first presidential nominating contest, New Hampshire holds the nation’s first primary election. You don't tick off New Hampshire by not showing up unless it is absolutely necessary.
I am up early today, 4:30 am, and I still can't find why he was called to the White House. I will keep looking.