It's an odd phenomenon that is receiving more documentation in recent years. The going theory is that occurs as a defence mechanism in corals when exposed to dratamtic changes in lighting and dkh (CO2 may be involved in the process). Whether this is associated with a "stress response" in terms of stunted growth/increased susceptibility to disease/ calcium reuptake is yet to be determined. It's been known to occur is species of sps (millies/monties/horns), coralimorphs for the godspawn/bubble mushrooms, lps such as your encrusting acan and symphyllia. more often than not the bubbles become axial coralites in sps (the nuby bits) or go away on their own and return to regular skin tissue.
Either way, it's certainty a trending phenomenon for hobbyists as collectors items as well as topics of interesting abnormal behaviour. I've included a picture from another source showing an sps with bubbling tissue incase anyone's interested in seeing sticks with bubbles.