I think it has been pretty much a standing reality that our presidents deserve impeachment. Ever since the Whisky Rebellion there's been a reason or two for a senate trial. Now there reasons abound for impeachment today, generated two parts incompetence and no small measure of self-absorbed arrogance, but the oppression of 'tea party' and 'patriot' groups via the IRS have a funny flavor to them- logically they would favor the Republicans far more than Obama.
The organizations targeted appear to be the sort of organizations that, should they field any candidates, would take votes away from Republicans. Barring their ability to field their own candidates, these organizations are of the sort that could influence Republican candidates and/or office holders locally, potentially reducing the Republican leadership's control.
So why would a Democrat administration risk impeachment to neutralize the thorn in the Republicans side?
Maybe they didn't.
The results would have favored the Republicans politically, and favored your standard American bureaucrat institutionally, so it isn't hard to see various IRS employees seeing this as a righteous and just cause (i.e. self-preservation), since most of these organizations are likely to view the abolition of the IRS with some approval. Both the Democrat and Republican bureaucrat would find all sorts of justifications for this vendetta. It would not take much to put them on the hunt.
Despite the absolute drivel coming out of the White House on various things, this is one of those things that I tend to think they would know- that, politically, the enemy of their enemy, at least in terms of the vote, is their friend. Obama and co. do have a somewhat successful history in terms of winning elections, so one could expect at least some sort of feral cunning in which they would actually encourage more discontent with mainstream Republican idiots like Romney.