Another way it can automatically unfreeze is if you are using a bookmark to your garden which you saved while performing an action. For example, if you unfreeze your garden and then save the URL to use as your bookmark, your garden will automatically unfreeze every time you access it via that link. This is also the case if you saved the bookmark immediately after accepting a gift.
To be sure the bookmark you use to access the game is "neutral" and doesn't perform any actions, make sure the URL at the top of the page looks like this before you save it (with your own garden ID in place of xxxxxxx):
https://www.fairylandgame.com/fbfairy/fairygarden.php?id=xxxxxxx
