Safety Concerns you are making your assumption on the basis that the overtime is always wanted by the engineers, plenty of places I have been it's the management twisting the engineers arm as they have no cover, a/c late off check etc. Perhaps if the QA dept's looked at the shift/ station cover on their annual audits and addressed it then the overtime would decline but that would require them to put their neck on the block which rarely seems to happen these days.
Things are rarely are simple as they seem.