For a forwarder a rollover often surfaces after the fact: the vessel has sailed, the container has not, and the desk is left re-planning the onward delivery and telling the customer the cargo is now on a later ship. Common triggers are carrier overbooking, vessel schedule changes, customs or regulatory holds, and missed cut-offs, the routine example being a container with no VGM filed by the carrier's cut-off. The rolled container waits at the port for the next available vessel on the same route or is rebooked onto another service. A rollover is not the same as a blank sailing, where the carrier cancels the sailing itself: in a rollover the vessel sails, and your container is bumped from it.
Glossary
Container Rollover
The carrier's act of taking a booked container off its confirmed vessel and moving it to a later sailing: the ship departs without it, and the container stays behind at the port.