When someone reviews a patch in Gerrit (e.g. CR-2) and then removes themself from the list of reviewers, that CR-2 is removed too. Numerous examples exist (gerrit IDs): 190717, 120811, 175394, 175192, 171562, etc.
While we might not fix this in Gerrit (to be superseded by Phabricator's Differential) it's worth to check that this will not be the case in Differential at least.
(Filed as one outcome of T113378)