Grendel's mother avenges Grendel's death at the hands of the humans. In Herot, the humans are celebrating the end of the twelve year blood bath from the feuds with Grendel. However, their revelry abruptly comes to an end when Grendel's mother comes to Herot to avenge her child's death. She snatches Aeschere, Hrothgar's closest advisor, and slays him. As a warning to the rest of the humans and as revenge for Grendel's claw being displayed after his death, she leaves Aeschere's bloody head mounted on the cliff. When the men go searching for Aeschere, they stumble upon his bloody head.

Grendel's mother reignites the deadly feud with the humans as she avenges her poor child-monster's death. While Grendel initially never means to hurt humans, Grendel's mother is motivated by vengeance.

The previous poster is correct; Grendel's mother comes for revenge. I want to add just a few interesting details.

Mother comes for revenge only after everyone has already celebrated Grendel's death at the hand of Beowulf, including Hrothgar, who showers Beowulf with gifts and says that he's ready to adopt him. (I think this last detail is what makes it possible for Beowulf to later assume the position of king of the Geats.) She's a much worse adversary than Grendel, of course, and the passages describing her underwater lair and Beowulf's battle with her (e.g. the blood bubbling up that is seen by Beowulf's men as a bad sign) are among the best in the entire work.

To me, Beowulf essentially shows here that he's not just fulfilling the letter of the agreement with Hrothgar. He had already killed Grendel and, by all rights, could have moved on with his rewards, leaving someone else to deal with the revenge-seeking mother.