It’s the glue
Get rid of the glue.
If you really, really, need to secure pieces so they do not slide around by the time you get them into the kiln, then use cheap, unscented hairspray. I keep some in a small bottle with an eye dropper. However, I hardly ever use glue.
Just remember that the more “contaminants” you introduce into your firings, the more likely you are to have issues.
Jester’s Baubles Fused Glass Designs