Probably arson.
https://www.kentucky.com/news/state/kentucky/article249828288.html#storylink=mainstage_lead
This bridge was in Washington County, in the central part of the state. It's one of the few in the state I've never visited. Most of the state's covered bridges are concentrated in the northeastern part of the state.
Back in 1980 or 81, the Sherburne Bridge crossing the Licking River between Bath and Fleming counties was burned in an arson fire. The road carried KY 11, the main route between Mt. Sterling and Flemingsburg/Maysville. I was in college at the time and wrote a story about the incident for a journalism class. I crossed that bridge many times as a child.
Sad. Hope they catch the arsonist, if indeed that was the cause.