Why am I getting arcing on my neutral bus in a sub-panel?
Loose lug nuts can lose you a race.  Loose neutral lugs can lose you your house

The arcing and failing branch circuit neutral connections in the subpanel are likely doing so because they're loose (i.e. insufficiently torqued).  Retightening them isn't enough though -- you will need to get a torque screwdriver in, turn the subpanel off, undo all the neutral screws, cut the burnt ends off the wires and restrip them, then reinstall and torque the neutral lugs to specification with the torque screwdriver.