Well, I emailed MM about this, and here is the response:
Quote:
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A homeworld will never riot. Their anger will go up but it maxes out at 80% (where 100% is required to riot). We had to impose this during beta testing because a newly started empire would have its one homeworld riot and that player's game would effectively be over.
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A planet designated as your HW cannot ever riot. 
But, if the HW is glassed, then recolonized, it can then be capable of rioting, as it is no longer a HW.