Being swooped by a "gulp" of swallows

While hiking in Pole Canyon Ranch in the Texas Panhandle, we came upon a "gulp" of swallows that I think were feeding their babies. They were attacking a small area of wet sand in the creek and flying up to the nests where tiny beaks were sticking out accepting a meal. We slowly approached, and if you listen, you can hear when a few of the swallow calls changed. All at once the entire "gulp" swooped right at us. It was unnerving. After we passed, they went back to attack another patch of sand and feed their young.