History
During Ultron's global take over, Banner already had control over the Hulk and didn't want him to resurface under any circumstances before isolating himself from the rest of the world. When the second generation of Avengers wanted his help to defeat Ultron, he refused, until the group managed to attract Ultron's attention. In fear of the situation, Banner turned into the Hulk once more, destroying Ultron before fleeing with a newly reunited Betty Ross.[1]