Nesting birds are known to dive bomb any potential predator that is near their nests. That means they don't have to know that squirrels are going to rob their nests before attacking the squirrels. Nevertheless squirrels are known to climb up to bird nests and eat the eggs and the nestling birds.
Squirrels have been known to rob eggs from birds' nests, i'm assuming this was the scenario and the robins were defending the eggs and/or the nestlings.
I should think the squirrel was getting too close to their nest and babies.
Saw two of them attack a squirrel this morning
