PMDD has been recognized and studied as a phenomenon for a while in the medical community, but it has really been fully accepted as a psychiatric disorder in America in DSM-5, which came out in 2013. We talk about it as a psychiatric disorder because the symptoms are expressed through the brain and behavior. The generally-agreed-upon cause is abnormal brain sensitivity to normal hormone changes across the menstrual cycle. 

Calling it a psychiatric disorder doesn't mean it does not have a biological basis -- there are many studies on how hormone reactivity in the brain and gene expression differ in females with PMDD.