Some materials will partially or completely combust at fusing temperatures. This gives off gasses which expand and blow big bubbles from under the glass.
Some kiln washes, especially for ceramics, give this problem. If you believe this is the cause, try a different brand of kiln wash or pre-fire the kiln shelf.
Sometimes organic materials have been introduced accidentally or purposely onto the shelf. Either clean the shelf of the old kiln wash, or support the glass on beads, or frit to allow the gasses to burn out before the glass slumps to trap the gasses.
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