The Question and Answer section for The Epic of Gilgamesh is a great resource to ask questions, find answers, and discuss the novel.
Anu, Lord of the Firmament, is the father of the Sumerian gods. Ishtar appeals to Anu after Gilgamesh turns away and refuses her advances. She forces Anu to send the Bull of Heaven to kill Gilgamesh by threatening to unleash the dead.
Humbaba's magic confuses Gilgamesh. It disorientates and terrifies him.
Gilgamesh rejected her sexual advances..... something that insulted her enough to request the Bull of Heaven be sent to destroy him.