So, we all know that Calypso cursed Annabeth after Percy left her isle. Buy why? I have a theory.

Calypso was originally characterized as a neutral third party in PJO, she is described with sort of a quiet acceptance of her situation on the Isle. She is intended to show the gods folly in relation to their past to Percy. He even left her isle with good feelings between them, she accepted that Percy had to save the world and Percy promised to free her later. She even gave him some moon lace - this does not sound like she is bitter enough to curse him. So what changed?

I have a theory that Hermes went to Calypso's isle and put it into her head that Annabeth was the one barring Percy from rescuing her. It would explain why she had a change in attitude about Percy's stay on her isle and why she suddenly blamed Annabeth enough for it to curse her. Hermes would want to hurt Annabeth in some way too. He hated her for the longest time after TLO because he blamed her for not running off with Luke and saving him from Kronos. They didn't make up with her until after the Staff of Hermes either, so there is a good few month window for him to do it. He likes to make mischief too, it would make sense for him to stir something up.