The leaked data and hacked website are most likely in reprisal for last Thursday's BART shutdown of cell phone voice and data service in four underground stations, Civic Center, Powell, Montgomery and Embarcadero. BART shut down service in those stations in anticipation of a possible protest over a July BART officer-involved shooting.
Instead, what BART got was a different protest on Monday of this week. Anger about the shutdown spilled over from groups like EFF to the public, to Anonymous. And, Anonymous not only hacked two websites (myBART.org and californiaavoid.org) on Sunday, it encouraged followers to protest at the Civic Center BART station on August 15 (which they did).
The past tense should be used to refer to the leak BART police data, however, because the original pastebin link for the data appears to be dead, now. While Anonymous appears to be a good candidate for the hacker group involved, a Tweet from one of their Twitter accounts clarifies why it isn't clear: Anonymous is not unanimous.
What that means is something that we've mentioned before, but that is generally glossed over by mainstream media. Anonymous is not a tightly knit set of hackers. Instead, the name Anonymous refers to a loosely-tied-together organization that consists of groups of hackers. These groups don't necessarily work in concert with each other, or even agree with each other.
That explains why there appears to be no centralized Twitter account for Anonymous. There are even multiple accounts discussing these BART protests, although central to them is @OpBART.
In addition, many of the recent hacks that Anonymous has performed are part of its AntiSec campaign (with LulzSec) which focuses on corruption in big business and government. Its response to BART can be seen as part of that, and the two groups have asked for help from their followers, so it's possible neither group was responsible for the BART POA hack.
The FCC has opened an investigation into BART's shutdown of its cell phone service. Meanwhile, Anonymous, via @OpBART, has announced another protest to take place at Civic Center BART station, on Monday, August 22nd, at 5 p.m. PST.