> The National Labor Relations Board [NLRB] has long taken the position—which the parties accept—that the NLRA [National Labor Relations Act] does not shield strikers who fail to take “reasonable precautions” to protect their employer’s property from foreseeable, aggravated, and imminent danger due to the sudden cessation of work.
> the Union argues that “workers do not forfeit the Act’s protections simply by commencing a work stoppage at a time when the loss of perishable products is foreseeable.” If the mere risk of spoilage is enough to render a strike illegal, the Union insists, then workers who deal with perishable goods will have no meaningful right to strike.
> But given the lifespan of wet concrete, Glacier could not batch it until a truck was ready to take it. So by reporting for duty and pretending as if they would deliver the concrete, the drivers prompted the creation of the perishable product. Then, they waited to walk off the job until the concrete was mixed and poured in the trucks. In so doing, they not only destroyed the concrete but also put Glacier’s trucks in harm’s way. This case therefore involves much more than “a work stoppage at a time when the loss of perishable products is foreseeable.”
> the NLRA does not impose a legal requirement that workers give specific notice of a strike’s timing [but] the Union’s choice to call a strike after its drivers had loaded a large amount of wet concrete into Glacier’s delivery trucks strongly suggests that it failed to take reasonable precautions to avoid foreseeable, aggravated, and imminent harm to Glacier’s property
So I think the Supreme Court is quite reasonable in saying that the Union could be liable for the destroyed concrete in this case.
(I don't have an opinion on the jurisdictional question of whether this should be decided by the NLRB vs. a state court.)