Why is Lisa Jackson, EPA head, stepping down?

[Enrique]

3. and Keystone, according to this person

http://www.buzzfeed.com/rubycramer/top-obama-environmental-official-departs-frustrat

(...)
Jackson "left as a matter of conscience," said Jeff Tittel, the director of New Jersey's Sierra Club chapter and a longtime friend of Jackson's. The EPA Administrator "has too much principle to support the pipeline, between the climate impacts of it and the water quality impacts of it."

(...)

A former senior administration official confirmed that disputes over Keystone were central to Jackson's alienation from the White House — though a third source close to Jackson said she also had personal reasons to move on from the Obama administration, a normal step for a cabinet official who has served four intense years.

"It was all about Keystone for the last 16 months," said the former senior Obama administration official, who said Jackson's opposition to the project — and her defeat in internal arguments — meant that her colleagues had assumed she would leave after the November election, before she would be forced to work on any element of the implementation of Keystone.