In any sphere of work there are those workers who are there because it is what they want to do, as well as those who are there purely 'for the money'.
If the money is meagre, (and inadequate to support accommodation and food) then workers will struggle for extra hours (with associated tiredness, especially among those who are not there as their vocation).
I recall from personal experience a period when I was required to work considerable overtime - there was little opportunity to spend the income, and, although I was there by choice as my chosen occupation, I eventually became disenchanted because of the lack of 'a life' outside my work.
Maybe the injured party was not dedicated to his job, but merely struggling to get through his tasks (including driving) with the ambition of getting a rest period?
Under such circumstances it is easy to become careless (as seems to be the case).