This should not be surprising at all. After Bass' performance as a starter this season, no agent worth his salt would advise his client to accept the one-year option especially when considering the lack of depth in this year's pool of free agents.
Also, in the comments Bass made after game 7, I'm pretty sure he said something along the lines of "I hope to be back" or "I'd definitely like to be back" NOT "I'm not opting out." That means he'd like the Cs to give him a nice long contract offer. Knowing DA that's not likely.
Bass is a very good role player for a veteran team. My gut feeling is besides PP and Rondo this team will be very very young next year.