why do seagulls chase seahawks ?

Answer 1

Most birds view hawks as "danger" the gulls are likely chasing off this "danger" as to avoid a possible attack. Another reason is that sea hawks aka ospreys feed primarily on fish. Some of the larger gulls will commonly attack other birds, including large raptors to try and steal their prey, in the case of the osprey, basically attack it and hope they drop the fish to escape.

Answer 2

they are trying to upkeep their territory.