I have heard both of these completely different theories
1. They bubble in an effort to shade themselves in high light situations, and don't bubble to open up more body surface area in low light.
2. They bubble in an effort to expose more body mass to light in low light, and have thin tentacles in high light to expose less flesh to light.
Totally oposite theories. Pretty sure nobody has any clue as to why this actually happens. Lol