Read about it here.
It seems to me that Jason Ewert knew the grading system well enough to be able to identify flaws that could be a) micro-fixed to improve grade .2 or better, and b) shaved away undetected. From the few examples that have gone public, it seems as though books whose primary flaw was slight fuzz on the spine corner(s) were good candidates for trimming.
Since the book in question was graded after the last Ewert trimming scandal, CGC obviously still cannot detect this trimming with consistency or accuracy.
One shudders to think just how many of these trimmed books have passed through CGC undetected.