Slave revolts

Holy shit. Playing as Rome and I'm not too far into my campaign, maybe 15-20 turns. Now public order isn't great which I'm not too worried about be cause the rebel armies are a joke. But good god the slave revolts are ridiculous. The fought off the first one with my garrison, but the second one was 2000 strong right off the bat and the decimated my garrison and a small army i had near. What causes this? is it related to public order or something else?