For those like myself not familiar with malignant hyperthermia, if in fact the cause. From the Mayo: (excerpts)
Malignant hyperthermia is a severe reaction to certain drugs used for anesthesia. This severe reaction typically includes a dangerously high body temperature, rigid muscles or spasms, a rapid heart rate, and other symptoms. Without prompt treatment, the complications caused by malignant hyperthermia can be fatal.
In most cases, the defective gene that puts you at risk of malignant hyperthermia is inherited, though sometimes it's the result of a random genetic defect. Genetic testing can reveal whether you have an abnormal gene. This genetic disorder is called malignant hyperthermia susceptibility (MHS)
Malignant hyperthermia susceptibility (MHS) is caused by a genetic defect (mutation). The abnormal gene increases your risk of malignant hyperthermia when you're exposed to certain anesthesia medications that trigger a reaction. The abnormal gene is most commonly inherited, usually from one parent who also has it. Less often, the abnormal gene is not inherited and is the result of a random gene mutation.
In most cases, no signs or symptoms of susceptibility to malignant hyperthermia exist until you're exposed to certain drugs used for anesthesia.
In rare cases, people at risk of malignant hyperthermia have shown signs of a reaction after intense physical activity during excessive heat or humidity, during a viral illness, or when taking statin medication.
If you're at risk of malignant hyperthermia and you don't have a serious reaction during your first exposure to certain anesthesia drugs, you're still at risk if you receive these drugs in the future. Other anesthesia medications that don't trigger a reaction can be used instead.
https://www.mayoclinic.org/diseases-conditions/malignant-hyperthermia/symptoms-causes/syc-20353750