It sounds like it was part of a whole "snowballing over-runs" syndrome, i.e. the initial over-run caused drivers to run out of hours, which delayed getting the trains shifted when the work needed them to be, which led to a vicious cycle of more over-runs and more drivers running out of hours and more delays moving the trains and so on. Not the only factor, of course, but it illustrates how problems pile up.
And that the initial problem wasnt caused by a lack of drivers!