The Jaffa operate their rulers' weapons, ships, and other technology. Their rulers train these subordinate forces and depend on them to do the work because they distrust one another. That arrangement gives the Jaffa the practical means to rebel. Their ability to turn against their rulers should cause little surprise within the story.
Notable Point · 1:55:55-1:56:29
Why arming the Jaffa makes rebellion possible
Stargate's rulers train and arm the same subordinate forces on which their power depends.