Why does Grendel's mother attack Herot

"Grendel's Mother's Attack. Grendel's mother, unknown to the Danes or Geats, is plotting to avenge the death of her son. After the celebrations are over in Heorot and everybody is asleep, Grendel's mother appears out of her dwelling place, the swamp. She enters the hall where the warriors are sleeping."

Because Herot killed her son, Grendel.