Looks like it's mostly caused by some form of head trauma. From the article:
Researchers suspect it is caused by dysfunction in the brain network that handles facial processing, though they don’t fully understand what triggers the condition. Some cases have been linked to head trauma, stroke, epilepsy or migraines, but other people have PMO without obvious structural changes in their brains.
NBC News Article on the disorder: https://archive.ph/usQIr
Looks like it's mostly caused by some form of head trauma. From the article: