The article sensationalizes the topic by putting "voluntary" in quotes. Per their contract, it's not voluntary if no one volunteers, it is mandatory, and then

The article sensationalizes the topic by putting "voluntary" in quotes. Per their contract, it's not voluntary if no one volunteers, it is mandatory, and then people are chosen by seniority and classification. So yes, people are being forced to work overtime, and the article conflating it with "voluntary" is misleading. Of course, if requiring overtime is to such a point where they are threatening legal action, wouldn't it be easier to hire more people?