It seems that in recent years, Steinhafel tightened the reigns over his exec staff, not allowing them any freedom of thought. The creative leeway they once had gave way to rigid performance metrics. This led some of Steinhafel's staff to take their grievances directly to the Board, complaining that in the wake of the recession, Target's offerings had become more commonplace to compete with Walmart. Target was losing it's "hip vibe", with fewer new product offerings.
So several key Target executives met back on May 2nd, and afterwards delivered a pointed message to the Board: if Mr. Steinhafel didn't leave, then others in their ranks would. You can read the entire WSJ article here on their site.