Its not the temperature that's the problem.
A magnifying glass will burn an ant as well if held at the right angle on a sunny winters day as a warm summers day
I just wanted to add that I had the same thing happen on my V-Strom with an aftermarket Shield, it didn't start happening until December even though I have the shield on all throughout the summer (100F+) here in Texas.
The bike was always parked the same facing north at work everyday, it wasn't the temperature but the low December Sun that worked its magic.