A year has passed since the moment Cas proclaimed that he is the New God. A "better God" who would destroy you in your spot if you did not profess your undying love to him.
But Cas failed to take into account that the power he was housing was not permanent. Even something as small as making Raphael explode used up some of his power—even if it was a small amount. Castiel started performing acts that are meant for God alone.
However, he was not omnipresent. He had no omniscience. He was not omnipotent—even though he was close. And most of all, he was not omnibenevolent. In his time of power, he showed more flaws in his character than he ever did as an angel. He made choices that make soulless Sam pale in comparison.
The angels followed without question. The demons, under Crowley's command, remained in hiding. The human race could not hide, though. But they did not bow down to Castiel, either. In fact, only a few hundred thousand knew of Cas' existence. But not a single human bowed down to him. The Winchesters made sure of that.
Slowly, but surely, Castiel started using up his power. He never felt a single change—his brain was too corrupted with power to notice anything else. He was a cruel god. At first, after they escaped (don't be shocked; they're the guys who saved the world), Sam, Dean, and Bobby tried to give him the benefit of the doubt. But the things Cas started doing were even more shocking than Sam's memories.
By the time the year was out, as far as Bobby and the Winchesters were concerned, Cas was even worse than Lucifer. After all, he got what Lucifer wanted in the first place: God's job.