I'm just guessing, but I'm of the belief that the air is squeezed out of the resin. The resin starts as a fluid, with air dispersed in it (rather than dissolved, like CO2 in carbonated water) but under pressure I imagine the resin closes up most of the space between molecules and the air would be forced out, since the resin itself probably can't compress, or only slightly. I'm guessing that the compressed resin behaves like a solid, while the surrounded air remains fluid, so the air and the resin would separate, since the entrained air would get pushed from a higher density medium (the resin, which is rapidly closing in) to much lower density (the air in the tank). So the difference in density is the key factor, and the increased pressure acts like a centrifuge in the sense of the constituents being arranged by density. Experts in resin casting don't by definition have to know the physics behind their observations. The tiny bubble hypothesis fits well enough with their observations that the question of its veracity is irrelevant. A master baker can misunderstand precisely how yeast causes dough to rise as long a he or she understands how the process behaves.
