There could be a couple reasons. If you find yourself reattaching the bracket as well, it may not be well secured to the wall, and you’ll need to reinstall the rod and its hardware. Another reason is that the curtain rod is overextended, and there’s too much weight in the middle of it. In this case, you’ll need to invest in a longer rod to prevent it from collapsing in the middle. If you have cats, it’s always possible they’re playing around the curtains. As small as they are, cats can add enough weight to a curtain rod to cause it to fall.