The job is a lot harder than people think. Metro TOCs (not just Southeastern) tend to suffer more because they encounter more issues. Multiple train lengths, Both side door releases on a regular basis, stopping at more reds, multiple routes in quick succession, multiple changes in linespeed in a short space, bored due to repetition, intense diagrams, various fatigue issues, multi traction. Add to that the normal issues with shift work and other aspects of train driving.
Even on a national level. You are at your highest risk of an incident in your first year. Not forgetting that there is a rather draconian approach to safety. 'Incident' is a bit of a swear word but it has a very wide net to cast. Pretty much everything is considered an 'incident' and will go on your permanent record. The railway is very unforgiving.
Imagine doing a job where you cannot make a mistake. There is no tippex, no delete button, you cannot reprint a page because you missed a paragraph or spelt something wrong, you can't take the next roundabout, and if your only a minute late you still may need to fill out reports.