Rather than speculating, it always makes sense to call the carrier, in this case DL. This may be a ticket problem, a document issue, a security issue or nothing at all.

If a ticket problem, this is an opportunity to get it fixed now rather than when it may be too late or stressful at the airport. If it is simply the need for a document check, you know that you need to allow more time at the airport and can plan accordingly. If you get some vague answer, you know that it is "SSSS". The last are sometimes entirely random and mean that it is just your unlucky day. Thus, the fact that you have not had it before means nothing.

Finally, if it means nothing, it means nothing.