What happens tykstil is this: The shop installs the cog with a regular chainwhip, and they get it pretty tight. Next they install a lockring to the correct tightness. The problem is, your pedalling the bike up a big hill puts more force on the cog than a chainwhip can. So, when you get to the top of the hill, now the cog is tighter. but that makes it farther away from the lockring. So, your lockring has effectively loosened. Next time you exert backwards pressure on the pedals, say by skidding, the cog may spin away from the hub and slam into the lockring. After that, a hard acceleration will spin the cog the other way, slamming it into the hub. This back and forth violence will quickly strip threads.
Your local shop may be to blame for what happened but it's hard for me to say based only on your description.
If your hub is ****ed, that sucks but you arent the first person that this has happened to. The way to prevent its happening in the future is to get a lockring tool, mash up a hill, and tighten your lockring but good at the top. (No, you don't need a chainwhip for this). Your cog will now be as tight as it can get, and secured in place by the lockring.