I have a microwave kiln also, but mine is the larger microkiln. I’m wondering if you have some metal on the bottom-inside of the kiln, maybe imbedded in the base? I had the same thing happen to me when I put a brass filigree between glass layers (duh, I know). Within 10-20 seconds of starting the microwave I had a mini lightening bolt in the mic kiln which scorched the base, and melted a dime size area of the glass. It made a chinking sound which prompted me to stop the microwave and check the glass.
Hope this helps.