It looks like it is the PT itself that has decided to dump Dilma Rousseff from the presidency.
The presidency had asked PT members on the Ethics Council of the Chamber of Deputies to support the Eduardo Cunha, the President of Chamber. The man is completely rotten and everybody knows it BUT he had the power to accept or refuse a formal request to launch the impeachment process. They voted against Cunha and, predictably, the latter immediately answered by launching the process.
More discussions and negotiations are in the works, but Rousseff's position is much weaker as a result.
What could explain the party's attitude? I see two things, that may overlap.
1) The country is going through a massive multidimensional crisis. The economic situation is dire (growth for 2015: -3%; inflation >10%; unemployment bordering 10%, double of last year); a national health emergency could soon develop around the Zika virus; and the Petrobras corruption scandal is reaching ever deeper into the political establishment. Except for the latter--which is getting ever closer to Lula himself--leaving someone else to deal with the mess would increase the chances of a victory in the 2018 presidential crisis, where Lula, if he is still standing, would have the best chance of winning among all possible PT candidates.
2) Many in the party's base--the so-called social movements--are up in arms against the current Finance Minister's austerity package, and deeply critical of Dilma Rousseff's giving cover to it.
The first is most likely, but don’t underestimate the second, as Lula could himself be thrown under the bus by his various close friends who are currently negotiating plea bargaining deals with federal prosecutors. If he falls, it will be civil war inside the party, and looking good right now may pay off.
For the details (worth translating), see this Folha de São Paulo article and, for background and a very careful presentation of the next steps, this, from the Financial Times.