I've been on crowded trains in such a situation and seen people suffer from dehydration, exhaustion, panic attacks etc. I can well understand why having been in that situation before, some people may try an extricate themselves by any means possible.
.
I don't know how crowded the trains people decided to evacuate were. My train left London Bridge with plenty of seats to spare just a few minutes after the accident, so I guess the trains just in front were not overly crowded either.
In any case, in an emergency surely other passengers should look after those who are suffering? I know this doesn't happen in reality, although on my (diverted) train a lovely lady passenger shared her birthday chocolates with other people in the carriage, and people were talking to strangers trying to work out the best way home, who could share taxis etc. This is most unusual in London and for a few minutes I thought I was back home in Yorkshire
If someone is dehydrated, surely one fellow passenger will have a bottle of water they can give? Exhaustion - well I'm not a doctor but surely if you're exhausted the best thing to do is sit down rather than go for a hike along the side of the railway. And again if someone's having a panic attack, I'm not convinced climbing out of the train and going for a walk in the dark is the best course of action. If someone is so seriously ill that they need to be removed from the train, a 999 call should be made. It will be much safer for BTP or railway staff to make their way to the train, with paramedics if necessary, than for someone who's apparently ill to go walking about in the dark.
Out of interest is there any evidence that those people who broke off trains were actually ill?
My neighbour tells me people broke out of his train
after the driver announced they were about to set off back to Lewisham. This suggests to me they may have been people from Hither Green who thought the walk from where ever the train was would take less time than going back to Lewisham and waiting for the bus...