Quickly, BPITT's starbond's TAG (trailing average gross) is an average of the final gross of his last 5 movies. For him, that is (Megamind + Tree of Life + Moneyball + Happy Feet Two + Killing Them Softly)/5 = 63.2 million.
When WWARZ delists in 4 week, Megamind will be dropped from the TAG calculation, to be replaced by WWARZ's gross at that point. Based on these early estimates of WWARZ's opening weekend, we can guess that it might delist in 4 weeks somewhere in the 135 to 170 range (if we're being really generous). A quick and dirty way to recalculate his TAG based on those numbers would be to find the difference between Megamind's gross ($148 mill) and WWARZ's gross (somewhere between -13 and +22 million), divide that difference by 5 (giving us -2.6 to +4.4), and then sum it with the current TAG. So his adjusted TAG when WWARZ delists could be between 60.5 and 67.6.
Since the range of possible adjusts for him is lower than his current price, people are selling/shorting him.