If memory serves, Oaxaca has always been a trouble spot in Mexico.
Your memory serves you quite well Fiddlstix. Perhaps not as much as its next-door neighbor to the south, Chiapas, but still quite a bit. The original Mexican "homegrown" drug operations depended upon Oaxaca for marijuana and opium poppy cultivation, making the state pretty much "off limits" to federal authority. But that also brought underdevelopment along with it, because it scared development away. And if you go very far back you will find that Oaxaca was the center of violent revolutionary activity going back to Emiliano Zapata.