yes it most likely is due to an airbubble in the glass.
ill be addressting this situation with production tomorrow
i will replace them if they explode, just send me a pic of them to ryan AT glasswearstudios.com
GWS glass is autoclaveable
an air bubble esp one that is not round can cause uneven expansion/contracting while heating/cooling. and galaxies have a tendancy to contain a non round airbubble.
these are only the 3 and 4th plugs i have heard of this happening since we released them in 2004, so thats a very low percentage.
i would like to know what color and sizes this is happening to, this would help us to figure out where the problem was in production of these. if you photograph them please include the surviving one if only one broke.