This happens for a few reasons. The first possibility is that your pie crust wasn't chilled enough. If the butter is too soft, your crust will melt before it has a chance to actually bake. Pie crusts must be chilled before baking. A second possibility was that the oven was at too low of a temperature, and similarly to the butter being too soft, the crust melted before it has a chance to bake. A third and more likely case is that the sides weren't supported enough. Make sure that the parchment paper is pushed directly up against the sides of the crust, and the pie weights are supporting them. This is crucial to ensuring your pie crusts edges don't sink in before they get a chance to fully bake and stabilize.