In Beowulf, why does Grendel's mother attack the Danes?

In the epic poem Beowulf, Grendel's mother attacks the Danes out of revenge. Beowulf had already defeated Grendel, the monster who had been haunting the Danes. However, Grendel--before dying from his wounds--returned to his lair; it is likely his mother watched him die. Grendel's death enrages his mother. She grows angry at the Danes, and determines to exact revenge on them for killing her son. She sneaks into the mead-hall, retrieves Grendel's claw (which the Danes had kept as a trophy), and abducts one of the Danes to use as bait to lure Beowulf to her lair. The bait works, and Grendel's mother attacks Beowulf on her home turf. Despite this advantage, Beowulf successfully slays Grendel's monster, decapitates the now-dead Grendel, and returns with his head.