It generally takes the feathers the same amount of time to grow back as it did for them to drop out over the moult, so a long moult means they look like a mangled feather duster for ages. Bullying will often be one bird pecking at one particular spot - feathers around the vent are often a favourite but back and neck can come close. A vigorous cockerel can also make them bare on the back and neck, particularly if he favours one hen above the others. Depluming mite is quite unusual.