Thats your automatic timeout protection kicking in. Its a perk that premium members get with your membership. When you're first put on vacation (once you've reached around 45 minutes left to make a move), 24 hours is taken from your total vacation bank. When you come off of vacation- that 24 hours is added to your game time.
Hope that helps explain it :)
I've been having trouble with my vacation settings automatically kicking in. It seems when I get down on time for my online games it kicks in and then I have to turn it off. Then I notice that my times are reset.
I don't know why it is doing this and don't want my opponents to think that I'm somehow trying to game the system.
Anyone know how to solve this?