Looks like maybe there were a lot of air bubbles trapped against the shell somehow. Did it have the air sack at one end? Because it looks like the wide end is the end that's uncovered, and there's clearly no air sack. Maybe the air sack burst and the air ended up all over the place somehow during boiling, or maybe there was a bunch of air between the membrane and the shell somehow?
looks like an Ebay Shipped Hatching Egg got into the boiling pot to me. I have no idea how those air bubbles got like that but the freezing theory is one I believe was a good guess... unless someone knows for sure that freezing would cause that. If I had to guess I would say it got shook up some how and the air sack got all over the place. What does the Yolk look like?
Odd that it's just the one egg. Could be it was right up against an especially cold area?
I also wonder if maybe it was some sort of issue where there were multiple tiny air sacs for some sort of internal-egg-factory reason. Were the pockmarks all lined with membrane like the air sack is?