You have to keep trying to leave.

The delves in Cyrodiil are not technically in Cyrodiil. I don't know how it works. If you enter a delve or Cheesemonger when Cyrodiil's population is full and with a waiting time of more than a few minutes, it takes time to reenter Cyrodiil from a delve or Cheesemonger. Keep trying and eventually you will be back in Cyrodiil.

Side-note and off topic a bit- If you have a scroll quest and the scroll gets recovered and placed while you are in a delve or Cheesemonger, your scroll quest will not be fullfilled. That's just another example of the delves not being technically in Cyrodiil.

Rule of thumb: Check your campaign's population and the estimated waiting time before entering a delve or Cheesemonger. If the waiting time is in the single digits on the low end- say around 4 minutes, the wait time to leave the delve or cheesemonger is not bad, otherwise, skip going into those places unless you don't mind waiting.