Paul had a long and complex relationship with the church at Corinth. He sent them many letters, only a few of which we have preserved in the New Testament. At one point, the relationship between Paul and the church at Corinth became strained. Based on what he says in 2 Corinthians, we know that Paul sent them a passionate and strongly-worded letter that they did not appreciate. They felt it was too much. It came on too strong.
If you ever wonder how Christian communities responded to Paul telling them they needed to be more unified or more humble, or what have you, 2 Corinthians shows us they responded like you would expect them to. They did not take it well. So perhaps to mend fences, he visits in person. Instead of making things better, the visit makes things worse. Things really fall apart.