Its not worth it for the predator animal to go after a bunch of smaller birds that are harassing it so they tend to leave. Its also done to steal the predators meal when they leave or to distract it while another bird steals its food. Also draws attention to them to prevent them going stealth, which can affect bjrds like owls, who tend to be victim to this type of attack from prey birds. This makes them leave because their main weapon is stealth and when their cover is blown, they will have a harder tome catching prey (including the mobbing birds' babies)
I feed my local crows peanuts every day. I really like crows, but I still find it incredibly amusing when the local bluejays - who are like 1/3 the size of the crows - chase them around and beat the shit out of them. It's not even a numbers thing, I've watched a single bluejay chase off the whole family of five crows.
The crows do the same thing to red-tailed hawks whenever they show up, but it's always five-on-one in those situations. Bluejays way more badass.
I live in Australia, we have a very small bird (called a willy wagtail, idk its actual name, that might be what its actually called) that would see that shadow, and then repeat 'i am the top bird' right before the half a handful of bird goes right after the eagle.