Tzemach Atlas has this post on the business of kashrut. I would only add that Rav Weissmandel was brought in to Rubashkin's stable of supervisions not because KAJ walked away, but because Rubashkin's market share depends largely on the perception of a higher standard of kashrut, higher than the OU alone. When KAJ agreed to supervise Empire along with Empire's existing OU certification, Rubashkin's edge was largely destroyed. Worse yet, all things being equal, Empire is widely considered by food experts to have the better product. Rubashkin's solution to this problem was to add the supervision of the Nitra Rav, Rabbi Weissmandel, who is widely regarded as having the strictest supervision in the business. (Rabbi Weissmandel is also reported to be extremely anti-Zionist.)