Reading between the lines I am guessing that normally finding people to volunteer for overtime is easy; in some cases the workers are offered it by seniority fi

Reading between the lines I am guessing that normally finding people to volunteer for overtime is easy; in some cases the workers are offered it by seniority first, as many blue collar workers see time and a half and holiday pay as very desirable. Hence the contractual concession. But in a heatwave it becomes a different story and now workers are noping out because it isn't worth it. And neither side may have seen this as a a possibility. They did not give up the right to strike entirely at all. And agreements like this are not uncommon for what are deemed essential services.