Root Cause: Unknown.
Our recommendation is to upgrade to the latest Automation Engine version 12, as there are many fixes contained between v9 and v12.
If the issue presents itself after an upgrade send Automic Support a set of rolling trace logs when the issue occurs in order to further investigate the issue.
Fix Status: No Fix
In the Event's "Attributes" tab, set the Tasks running parallel section to a maximum number of 1. This will stop further unwanted executions.