The labor-backed Economic Policy Institute calls the rise in the percentage of union jobs in the country "remarkable" given the current economic climate. Given the collapse of industry in the Midwest, in particular, it's a little jarring.
But it's easily explained, I think.
The increase overall due to an increase in the number of government employees who belong to unions -- and a corresponding slight shrinkage of the private sector relative to the public sector. During a recession, government often cuts jobs later than the private sector does. And -- non-union employees generally have fewer protections against job loss than unionized employees with contracts do. The lowest-hanging fruit in a downturned economy are non-union jobs. I think.
The Bureau of Labor Statistics release on the numbers is worth reading in full.