I'm no doctor, but I would hazard a guess that it is either a loss of balance due to sudden weight gain, or weakened joints due to sudden weight gain. She's already carrying 500 pounds every time she stands up, which is quite a load. That throws your balance off as it is. If you maintain a stable weight, you kind of know where your center of gravity is, but if you pile on 40 more pounds very rapidly, as she did, that center of gravity changes, but your body hasn't learned where it is yet. So, reaching for something that was reachable at 492 can throw you off balance at 532, and all those pounds come crashing down.
Or, her joints are starting to buckle under the weight, which is inevitable. This would be a much more serious situation than the first, but the first leads to this one anyway sooner or later.