I had a similar result on my last ride. It's flapping fabric, low humidity, and static electricity.

Here's the start of an easy to moderate paced ride, but I didn't wet the Garmin HRM strap with the usual two drops of water.

I saw a consistent 175-190 rate, then stopped and wetted the strap sensors, and it dropped to the expected 120-125 range. (My old Polar strap would show 220-230 when flapping in the wind on dry days. The Garmin rarely has these high heartrates if it's wetted beforehand.)