It could have been that the guard closed the doors, gave the driver the bell, but for some reason or another (s)he didn't move straight away, so the guard may have thought that they didn't have the signal, so being conscientious (made what's now a goof!) stepped out onto the platform to double check, only for the driver to then take power and go PDQ, leaving the guard on the platform!
They won't be the first & they won't be the last, it used to happen every so often at Surbiton and the guard would have to be taxied to either Esher or Hinchley Wood!
The reason the driver won't open the doors is two fold.
1. There was an "incident" where a driver activated the doors (with all the best intentions & it was proven there was no malice), which could have been very nasty indeed, so there was an instruction issued that drivers must NOT operate the doors of the train whilst in service, which is fair enough.
2. If it's rush hour the platforms are going to be packed, the DKS will be 99.9% of the time active in the centre cabs & most definitely not the leading one, possibly the rearmost should both the DKS be defective in the centre cabs (unlikely). Now bear in mind that drivers have now been instructed NOT to activate the doors from the previous incident and you can see the problem.
The driver's damned if they do activate them to allow passengers off, they're damned if they don't, the driver may risk a serious charge sheet if they do.
So what do you do? Me, ring control on GSM-R or ring a DM on your mobile (from the safety of the platform) and get authority to open the doors & detrain the whole lot and take the train ECS.