There could be a few reasons for this. The first is adding too much four. Make sure you fluff the flour and spoon it into your measuring cup and do not be tempted to add more. Second, there is a possibility you overbaked the brownies. If you are using a toothpick to test, you will end up with a lot of moist, thick crumbs sticking to it. A toothpick that comes out dry means they are overbaked.