Endless bad behavior , towards virtually everyone. Some of it can be attributed to her mother -- the unpaid bills for example. But Gabby is at least in part responsible for much of it, like the accusations about Excalibur. And Gabby is solely responsible for some of it -- ex -- the shabby treatment of Chow in Round One (allowing him to find out she was leaving his gym via the grapevine and only saying a hasty "goodbye" the last day when Chow wasn't even there), the recent catty remark about Jordyn.
The lack of especially good behavior -- I cannot think of one moment in the 2012 Olympics when gabby stands out as offering support to her teammates. She accepts it from others, but does not seem to ever offer it.
The bad behavior is consistent. It goes to character. Her decision not to do anything of any substance after the Olympics (no real training, no continued education) and to simply ride the fame train and play "celebrity" rubbed many people the wrong way, but ultimately gets chalked up (pun intended) to bad judgment and failure to understand that the money train stops running pretty quickly in this sport. But mostly it is the bad behavior towards others -- I do not think there is a bridge that girl has not burned at this point -- that has led to the criticism.