It would appear this effect is caused by the spinning motion of Ultima Thule

‘Flyby data has already solved one of Ultima’s mysteries, showing that the Kuiper Belt object is spinning like a propeller with the axis pointing approximately toward New Horizons,’ Nasa added. The principal investigator of the mission, Alan Stern, called the accomplishment “a technical success beyond anything ever attempted before in spaceflight.”

Ultima Thule is, as early occultation studies suggested with remarkable success, what’s called a contact binary object, composed of two individual objects fused together likely through impact billions of years ago — and it’s the first ever photographed up close. ‘Because of where it was formed and the fact that Ultima is not large enough to have a geologic engine like Pluto and larger planets, we expect that Ultima is the most well-preserved sample of a planetary building block ever explored. ‘In effect, Ultima should be a valuable window into the early stages of planet formation and what the solar system was like over 4.5 billion years ago.’

The New Horizons probe reached the closest point of its flyby at 05:33 British time on New Year’s Day, passing Thule at more than 30,000 miles per hour and coming within 2,200 miles of the surface of the 19 mile-wide object.
New Horizons makes a flyby of Kuiper belt object 2014 MU69, nicknamed Ultima Thule, at 05:33 UTC, becoming the farthest object in the solar system visited by a spacecraft. It is a contact binary.