Panniers stuck in tire spokes!

I ride my bike as primary mode of transportation everyday. Recently, I decided to start using panniers for groceries, to spare my shoulders. I hooked them on to the rack on my bike, and rode to Trader Joe’s just fine. Packed them up with my purchases, then rode home. I ALMOST made it all the way home, but about two blocks from my house I went up a ramp and the back corner of the bag on the left-hand side caught in my spokes! It sounded like my bike was exploding. I got off and started walking, but had to stop every few seconds to pull the pannier back out of the spokes. What is most likely to have caused this? It seemed like I had it installed correctly, and felt like the grocery weight was equally distributed in both bags. It also seemed like going up the ramp that led to the bike lane triggered it.

How can I prevent this from happening again, since I live in a neighborhood with lots of hills and don’t want to deal with this every time there’s an incline in the road?