They were still preaching works mixed with faith, I'm thinking.
I've heard the other gospel is referring to this stuff. Preachers trying to draw people back into the law. Just as they do nowadays, actually.
Acts 15:24 Forasmuch as we have heard, that certain which went out from us have troubled you with words, subverting your souls, saying, Ye must be circumcised, and keep the law: to whom we gave no such commandment:
I like what Paul says here about the reason his gospel was different. Christ in you, the hope of Glory.
Ga. 1:15-16 But when it pleased God, who separated me from my mother's womb, and called me by his grace,
To reveal his Son in me, that I might preach him among the heathen; immediately I conferred not with flesh and blood: