This all comes about as a result of Sen. Diane Feinstein's charge months ago:

Feinstein, in her dramatic speech on the Senate floor in March, said the agency breached the firewall to obstruct the committee’s investigation of the agency’s torture of post-9/11 terrorism detainees, a years-long effort expected to be partially declassified in the coming days or weeks. That investigation was itself prompted by a different coverup: the destruction of videotapes of brutal interrogations by a senior official, Jose Rodriguez.

Brennan's initial response to Feinstein's allegation was:

As far as the allegations of, you know, CIA hacking into, you know, Senate computers, nothing could be further from the truth. I mean we wouldn’t do that. I mean, that’s just beyond the – you know, the scope of reason in terms of what we would do,” Brennan said on the day of Feinstein’s accusation.

Colorado Senator Mark Udall is calling for Brennan to resign.