Paul had apparently never visited the church at Colossae but had heard that they were slipping into error. From the letter we can assume that this error was an early Christological problem, questioning who or what Y'shua is.
Some Christian commentaries presume that the problem was caused by Jews much as was the case in Galatia. This does not seem to be the case. Rather it appears that the new believers were getting trouble from friends in their former pagan religion. There seems to have been a desire to bring pagan ways into the church.