If you can notice, there are at least 4 users in the top 10 who have zero (0) total karma; probably, these users were karma nuked by the admin due to karma abuse. Their karma logs looks like parallel and getting karma least 2 karma points each day.
Promocodeudo
Salahmu
Roseline492
MainIbem
Yes nuked and looks like they had their karma issuance revoked a week or so ago, or stopped giving each other karma based on the logs (the latter I find less likely though).
Nice catch.
One more reason the open karma logs. 
There are probably some other abusers in this list, but they were not catch
This kind of abuse could be easily catch by the community with more transparency over karma data
Still not going to happen though, it would just lead to more retaliatory negative karma distribution. They were all clocked by admin about a week ago for reference sake.
Also as a warning to anyone thinking of giving negative karma to these users after they have been punished, that could be considered karma abuse.
Did anything happened that led to the deduction of their karma? Or is it a bug?
There used to be a Karma Abuser group which identified users who have had their karma issuance revoked but it doesn't exist anymore.
Not sure if this was due to technical reasons or because they ended up getting abused with negative karma by others over it.Edit: Turns out it was hidden to avoid the stigma after all.