I didn't. Someone weakened the barriers separating the demon realm from the human realm, and then started summoning them. Then demons got a hold of the devices they used to do so when the people who summoned them couldn't win, and used them to summon more allies.

[Why does he look so annoyed with him... He didn't do anything.

His cousin did. The Shomonkai cult did! He was just an innocent bystander...]