I noticed that my reputation is reduced by 10 with comment : user was removed. When can this happen ? What is the meaning of it ?
That means that a user who had voted on one of your posts has been removed from Stack Exchange, either by their own choice or more likely from violating the terms and conditions of site use. When this happens their voting totals are reversed.
As any votes they have made were done while they were in violation of the terms and conditions of the site they would have been classed as invalid and rolled back.
There are more details about how this works on the main Meta.StackOverflow site:
What does "user was removed" mean and why did my reputation change because of it?