Also not a geologist, and not familiar with local geology. Packing so much water in a very small channel really helps water erode anything. If i remember correctly, it's thought Niagara falls moved a few miles in a day. That seems terrifying to me, it's a long drop down. Waterfalls slowly work their way upstream, obviously. Less obviously, a little weakness lets more water though, compounding the abrasive effects. Which allows more water, which cuts away the land faster.

So yeah, i don't think the dryness had anything to do with destabilizing anything. Water is just really good at cutting. The dam created a huge reserve of water, so lots is available to cut.