I have an APEX Trigger that runs when a Task is created/updated, so it can update the related Case. 

Somehow, the trigger is causing the Case Assignment Rules to re-fire, but I don't know what.  Any thoughts?

trigger caseLastActivityDate on Task (after insert, after update) { Map<String, Task> tMap = new Map<String, Task>(); for (Task t : System.Trigger.new) { String whatId = t.WhatId; System.debug('whatId is ' + whatId); if(whatId <> null && whatId.startsWith('500') == true) { Case c = [select Id from Case where Id = :whatId]; if(c.Id <> null) { c.Last_Activity_Date__c = t.LastModifiedDate; update c; } } } }