"Soon" after Paul had been with these churches (Gal. 1:6).
While the debate was fresh about Gentiles becoming Christians.
While the Jerusalem temple and ceremonial rituals were in active operation (before A.D. 70).
Probably written from Antioch of Syria after Paul's encounter with the Judaizers there (Acts 15:1), and before the Jerusalem Coucil (Acts 15:2-35).
What was the situation in Galatia after Paul had left them?
The Galatian believers had been devoted to Paul and accepted his teaching when he was with them (Gal. 4:15-16).
Soon after Paul departed, other teachers arrived and began teaching things contrary to Paul's teaching (Gal. 1:6).
"But that's not what we were taught!" ... "Well, who taught you?" ... "Paul the traveling missionary."
The new teachers would say that the apostles in Jerusalem are the source of authority for the church, and that Paul must yield to them (since Paul was not one of the original 12 apostles) (Gal. 1:17-18).
The new teachers probably claimed to have been sent from Jerusalem and to be speaking with the authority of the apostles there. They taught that the Law of Moses was binding on believers and was required for justification (Gal. 5:4), especially the laws about circumcision and the Jewish ceremonial rituals (Gal. 4:10).
"The Jerusalem apostles still keep the Mosaic Law, and every one of them is circumcised. If you want to be on the same footing with the Jerusalem Christians, you too must be circumcised and follow the Law." (Gal. 2:3; 5:2; 6:12)
They accused Paul of being a "man-pleaser" (Gal. 1:10) who would change his teaching depending on his audience -- sometimes for circumcision and sometimes against circumcision (Gal. 5:11).
The Galatian believers began follow these new teachers (Gal. 1:6) and began to observe the rituals of Judaism (Gal. 3:1).