This post from the QB says that it lit inside the MW.
http://www.quiltingboard.com/main-f1...re-t77011.html
I've seen people report it both ways in the MW, after it's removed. Lots of people report that they used all cotton others don't know because they were gifts.
One post I read said they used Warm and Natural which is NOT 100% cotton, it has a Scrim. One other person in the same thread later on said they used 80/20.... oi.
There's a lot that probably needs to be tested to find out what's causing it but I agree that it's likely the left over left overs on the bags / bowls, or high wattage microwaves, or hot spots, time in the sauna, or some combination of factors, since so many people seem to report the same materials used and such different results.