What causes Grendel to attack the Hall of Herots?

Mutilation of the head of Grendel's father (who at the end is not his father at all) by one of the Beowulf's soldier.

Explanation:

While still a young man, Beowulf traveled to Denmark where for twelve years King Hrothard and his people were oppressed by the monster Grendel. At night, they would break into Hrothgar's magnificent Hall of Herots, kill thirty warriors at sleep and take them to eat. Hrothgar built a huge ballroom to host the many festivities where he distributed gold bracelets to his warriors and members of the congregation. Thus during these festivities, mead was drunk, many toasts were toasted, and bards sang about all the wonders of this world.