What sort of things could have gone wrong if the train had continued, once it was moved on to the correct part of the platform of course?
Anything could of happened. The Driver has had an incident and the root cause of that needs to be addressed before he/she continues.
Maybe they were drunk ? Highly unlikely. Hungover ? Again, unlikely but after a safety incident a TOC can medscreen a Driver/Guard 'just in case'.
Medscreens also pick up on... medical issues. Maybe the Driver was on medication and it was affecting their concentration level. All medications must be declared, and authorised, but the Driver may have taken something as simple as a cough sweet and hadn't realised it was on the restricted list or was making them dizzy.
Taking the Driver off may be to medscreen and that puts the passenger safety first. That's a good thing.
The Driver may have a history of incidents and the TOC made the decision that the risk to passenger safety was to great.
The TOC decision is made for safety
Other factors may have caused the incident. The Driver may of been fatigued. The Driver may have personal issues and hadn't realised they were in fact affecting him/her so decided that they were unable to continue.
The Driver may of been shaken up about having an incident and made the professional decision to not continue in service. Also keeping the passengers safe. A Driver who has an incident has the potential to have a second incident further down the track. It affects your concentration level and causes a distraction. Why take the risk ? If your distracted or discover your concentration level is reduced for any reason. Stop driving. Maybe the Driver just wanted a break.
Its the same advice when driving your car. Take breaks, pull over if tired, stop your journey. Yeah its not a car but the advice is sound and is also given to Drivers.
Drivers must sign/declare a 'Statement of readiness' each time they book on and it covers Tiredness, Medication, Alcohol, Routes, Traction, and other factors. If the Driver or even the signaller feels this may be compromised then the train is taken out of service.
Safety, Safety, Safety.