I was on a London - Cambridge fast on Thursday that was driven by one driver to Hitchin (where it wasn't timetabled to stop) and then another to Cambridge, and which had at least two drivers travelling as passengers.
I know nothing about operating a railway, so this is an ill informed rough guess. But I'm going to speculate that there's some kind of poor management / planning gone on there and that an ideal world would have put one of them in each of several trains. If whoever organised had had to also organise hotels for them heaven knows what would have happened.
Trains are stopping all over the place to pick up and drop off drivers. If a driver can't get a train then it is impacting another train, and another.. It's madness. I think more are getting round in taxis now, but that poses problems in itself - like where IS the taxi. Is it in traffic? can you get through to the driver? Can the driver get through to control?