The very people Paul hoped to pacify in Acts 21, were provoked by his conformity.
Paul understood that Old Testament law has no relevance for salvation ( See: Acts 13:38-39; 15:10-11, Galatians ch. 3 and ch. 4) The interesting thing about Acts 21, is that Paul actually agreed to go along with the request of James, the half brother of Jesus, and the other elders. I could ask, why Paul agreed to something he himself didn't teach, or agree with, but, then I would have to answer this question and that would take more than one blog post. However, there is a lesson in this.