I have always broken in my gloves with oil and practice. I decided to hurry this one along by using the suggested oven tip I have heard about in the past. “Oh, just put your glove in the oven!” I never believed them, because I feared it would catch fire. I thought I was wrong. My Easter was ruined today.
Edit: Here is the link that says 15 minutes at 350F: https://ecosports.com/blogs/vegan-athletes/how-to-break-in-a-baseball-glove
On second look, the burn marks are obviously only on the top surface so I’m pretty convinced you used a toaster oven. The difference is like standing in direct sunlight vs in the shade. When you’re in the shade the only thing heating you is the air, when you’re in direct sunlight the sunlight itself is heating you. You can also feel a similar effect when sitting around a fire - your skin facing in the direction of the fire gets toasted while the rest of you, or parts of you that are blocked from it, are cooler.
TL;DR you toasted your mitt