3. He's cooling off. Hasn't the last month been very hot in the south bay? What is the ambient temp in the tub on an 80 to 90 degree day?
4. He's decided he likes to soak. In the absence of any temperature issues or any health issues, it is not uncommon for morelia to soak. Snakes can change habits just like any other animal. It's possible he has just decided that he wants to soak.