I imagine the Bruins run is a factor in those decreased ratings; quite a few Bruins games were head to head with Sox games. The biggest factor, though, is probably the West Coast one. The Sox have played a lot more games on the West Coast this year than they did last year to date. I would expect those games have lower ratings. In other words, I don't think this data means much and will likely normalize.
While I think the above factors - the Bruins playoff push (and the Celtics, for that matter) and the front-loading of West Coast games - has a lot to do with the decreased television viewership, I think there are bigger reasons for the lower ratings (not necessarily in order):
1.) No Jerry Remy - Love or hate him, at least he's knowledgeable and complements Don well. With so many of Jerry's replacements having butchered the analysis and/or our eardrums, I wouldn't be surprised if more people were simply listening to the radio.
2.) Complacency - I fear that we've arrived at a near-MFY state where there's not as much urgency to follow the team as closely as over the past five or six seasons. This statement clearly does not apply to members of SoSH and other hardcore and crazy fans, but it may apply to the "fans of a lessor stature than I" and other casual fan circles that make up a large number of NESN's audience. It's tough to quantify the impact on ratings, but the expectations fans have for a team may play a role in how closely they follow said organization.
3.) The Team is Boring - I might get skewered for this statement (though I think many would agree): the 2009 Red Sox are a boring team to watch, both on and off the field. Other than Dusty, Papelbon with his fist-pump, and the insanity of Beckett's glare, who else stands out? Sure, Youkilis growls and Papi is still around, but the jovial, fun-loving, always-have-something-to-say players are absent from this roster. Theo has assembled a team of Jason Bays who don't say much, don't show much if any flare, and go about there business as if it were simply that: a business. Similar to my point above on complacency, I don't believe the team's character or lack-there-of affects how much the hardcore fan base follows the team on a regular basis. I do, however, think that the casual fans and even the media are not able to truly fall in love with this team - at least not yet. I think that the shift from Pedro and Millar-type personalities to Drew and Lowell-typle personalities may be negatively effecting the ratings.
4.) Other Programming - The Sox have also been competing with the likes of American Idol and other reality television, the other local teams as mentioned above, and everything else that the Internet and cable and satellite t.v. throws at its consumers. To me, this is the biggest reason for the ratings drop with everything else mentioned has only potential contributing factors. I'd like to know if ratings around the country are down and how the numbers across the board compare to the past few seasons.