Well unless the washer tab gets sheared off, the nuts still can't rotate.
I thought about the seals not being properly recessed, too, but it sounds like they've been tightened enough by now.
If the nuts ain't turning, the only way things can get loose is by the bearings changing their geometry. If this is the case, they're being overstressed; the hub design really isn't up to the loads being put on it. Wheel offsets especially put tremendous stress on hubs and bearings.