One possibility is that they have bought a lot of models.
Reputation point are also given to buyers, not just to sellers.
A second possibility is that they have gotten the points from referrals.
As long as their referrals upload a lot of models, the one who referred them also get a lot of points.
And thirdly. It is my understanding that it used to be a lot easier to get points when CGtrader was new. So if we are talking about old accounts, that might also be a reason. (i am not sure about this point though, i have not been around long enough to verify this)
so those are a few possibilities.