Not the first time and won't be the last. We've had one or two threads on this recently.
Some trains can quite easily leave someone behind if the guard (conductor / TM etc.) has a non-interlocked local door from which they can dispatch the train. This can apply to BR era multiple units (such as 455/8s) and HSTs, for example. All the driver may need to do is think they have heard "2" on the buzzer, take power and the train may be able to leave with the local door open and the guard on the platform.
The guard may also be alerted to something amiss occurring on the platform, and think it is preventing the train from moving even though they've given "2" on the bell or buzzer, and yet find the driver is in fact able to move the train. (In this case they should have pushed the bell or buzzer once - "1" is to stop - before making arrangements to leave the train and deal with the issue.
If there are power doors controlled by the driver under instruction from the guard, the driver could close all the doors but the guard's local door could shut as well (for example, if the local door switch is not active at the panel) or, if it is a slam door, it could be pushed shut. The driver, again, may think they have heard "2" and depart with all doors safely closed and interlocked, but with the guard on the platform.
If the train has DOO functionality as well as guard control, the driver could dispatch by DOO method in error, and forget they should have guard dispatch. Admittedly this is most unlikely on XC!
There have, from time to time, also been train wiring and other faults which have caused these issues.
Any of the above can occur quite suddenly and subsequently cause long delays and much paperwork...