"Around A.D. 61, Paul found himself in Rome under house arrest. It’s during this time that Paul wrote Ephesians, and it likely came about like this:

While Paul was under arrest, Epaphras, one of Paul’s coworkers from the city of Colossae (which was near Ephesus and started out of Paul’s ministry in Ephesus), visited Paul in Rome and shared the progress of the gospel in the region around Colossae. He also shared some particular problems that were troubling the church there which were undermining their stability in Jesus. So Paul wrote a letter to the Colossians.

Having written a letter to the Colossians, Paul decided that much of what he said would be valuable for Ephesus and some of the other churches in the region. So he dictated a second, more general letter, the letter we call Ephesians, not dealing with the specific issues troubling Colossae but passing on some related critical teaching to help establish and strengthen the other churches throughout Asia Minor. In fact, Ephesians and Colossians (and Philemon) are all sent at the same time and delivered by a man named Tychicus." 

-John Whittaker, Renew.org