Burned Clear Corners???

Today I got my clear corners in the mail and installed them. They look great. As I was doing the passenger side of the car I noticed the part that screws in was broken off from the main piece. This looks weird and it appears it has been burned. I have never used any heat around the front of the car and I tried to break the other one from the other side but wasn't able to with my hands. It takes a decent amt. of force to break this. I posted some pics of the break and what looks like melted plastic. Can anyone tell me what did this? I dont want it to happen to my new clears. Thanks.