Civil war

Not just a rebellion. Prelude to a rebel victory and declaration of independence.

Usually sparked by shortages of food or durable goods. The rebelling planet does not fix this on its own, so when you come to reconquer it, bring extras (or better, identify the shortage and fix it before the rebels win, then reconquer, then fix the resource flows). Also, pull any remaining undefected troops off the surface so they don't defect.