There are two overflow spillways built into the dam. The first one is a concrete path straight down the hillside to the river that can handle a high capacity. Next to it is another emergency spillway, which is just a cement edge to the top of the dam. Below that cement edge, it is just the hillside, and they knew if it would ever be used it would cause a lot of erosion to the hill.
It was the first cement spillway that failed, it developed a large hole about halfway down the side of the hill, and half the water rushed into the hole, severely eroding the hillside. They had to reduce the flow into that spillway, and in the meantime, the water level rose so much it went over the emergency spillway as well. And not only that, it went through a parking lot and over the top, eroding that too.
The big issue is that the concrete spillway that failed was known to have problems back a few years ago.