The NLRA 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.  Here, workers waited until the employer’s cement trucks were loaded with wet concrete.  Then they struck, allowing the concrete to harden and destroying the employer’s trucks.  That conduct was not arguably protected by the NLRA and so that act did not preempt the employer’s state law claims for destruction of its property.