Just had dinner with friends. They said a classmate just got diagnosed with a high blood pressure emergency. I didn't know what to say. Why would someone get that?

Many reasons. Probably the most common cause is when someone is treated for ordinary high blood pressure and stops their meds abruptly. Pheochromocytoma, a very rare tumor, is a known cause. New onset kidney disease and pregnancy-related eclampsia are addl causes. Spinal cord injury and interaction with a class of drugs called monoamine oxidase inhibitors are addl causes.
Emergency or high BP. Hypertensive emergencies are high BP readings (generally over 200/120) with bp-related damage to the heart, brain and/or kidneys. Importantly, high BP readings alone do not constitute an emergency. Hypertensive emergencies most commonly result from stopping antihypertensive medication or not treating hypertension or when another disease that aggravates existing hypertension (e.g., lupus) develops.