Why is the giant ice disk in Maine spinning?

There's a giant ice disk hanging out in the Presumpscot River in Westbrook, Maine that's garnered international attention, and rightfully so - it's an extremely rare phenomenon and this is one of the biggest ever recorded. Crazily, the ice disk is spinning completely on its own. Charlie brings in a paper that discovered just how ice disks like this can spontaneously rotate - and let's just say it's cooler than being cool. Read the paper and see the drone video here.