If you're using the newer softer strap it's because of static build up. There are a few ways to avoid it, you can soak the entire strap with water, or you can rub a dryer sheet on the strap/your shirt before you start running. I was having the same issue on the run and on the bike, though now these techniques seem to help.
I've been soaking the entire strap (not just the contact points) and it's mostly gone away. This was happening in both running and riding early on. It was going away later, but that was because I had been sweating enough to soak it. Wash the strap off too. Clean of the metal connectors between the strap and the battery section.