Currently admission controls don't have a globally consistent view of how many queries have admitted so different coordinators can over-admit if queries arrive at multiple coordinators in-between topic updates. Topic updates happen every 2s by default. Worse, admission control topic updates can get stuck behind large catalog updates.
IMPALA-3649 proposes having a globally consistent view of admission control, which will require architectural changes.
In the meantime we could reduce the problem by increasing the frequency of topic updates for admission control and separating them from the larger catalog updates.