The reason is that May penned a column about Barack Obama that was called racist. Here, via the Los Angeles Times, is a PDF of the column. The Times summarizes:
[May wrote] that the Democrat would hire the rapper Ludacris to paint the White House black (a reference to a pro-Obama song by Ludacris), and divert more foreign aid to Africa so "the Obama family there can skim enough to allow them to free their goats and live the American Dream." He joked that Obama would replace the 50 stars on the U.S. flag "with a star and crescent logo," an Islamic symbol, and that his policy on drugs would be to "raise taxes to pay for Obama's inner-city political base."May also wrote in the column, called "The (clarified) platform of Barack Hussein Obama," that Obama would change the national anthem to the "Black National Anthem."
The ouster comes not long after McCain chastised supporters for being insufficiently respectful of his rival in the wake of reports of increasingly ugly scenes at McCain/Palin rallies, where there have been shouts of "terrorist" in reference to the Democratic candidate as well as calls for him to be killed.
Meanwhile, another more prominent Virginia GOP leader, state party chairman Del. Jeffrey M. Frederick, has said both Obama and Osama bin Laden "have friends that bombed the Pentagon," a reference in Obama's case to his association with William Ayers. Frederick's comments drew criticism from Democrats and from the McCain campaign, though the chairman is standing by them.