Sounds more like it was hydro pulling him to the bottom. When waders fill up in deep water, don't they become neutrally buoyant meaning they're no worse than wet jeans and shirts?
I can see the bottom filling with air and flipping you upside down could be a serious problem.
I always wear a PFD when on my pontoon boat and really don't like wading in water that is over my head (test the depth with a wading staff if I can't see the bottom or just don't go in when I can't see the bottom).
Really sorry to hear about this and will pray for the family.
It's a good soberin reminder for us all to be careful out there.