Here's a good, clean statement from historian Juan Cole (Michigan):

The IRS scrutiny of Tea Party groups applying for tax exempt charitable status derived from a legitimate concern at the more than doubling of such requests after the Citizens United ruling, and a suspicion that the groups were backed by Republican billionaires intending to use them for politics, not charity.  It may be that the scrutiny was sometimes invidious, but it is not obvious on the surface as to whether the bureaucrats actually did anything out of the ordinary (left wing requests for tax exempt status were flat; if they had suddenly doubled presumably they would have attracted attention, too.)