Are you sure "cyclical" is the right term? I think neurosurgery, cardiothoracic surgery have been extremely competetive since their inception as specialties, and most of the other surgical specialties have been highly competive throughout. Surgery in general (esp. gen surgery) has been taking a hit to its competetiveness over the last few decades, but this is well correlated with the rise in popularity of lifestyle-positive specialties, like opthamology, dermatology, radiology.
There's basically been a long-term trend over the last twenty+ years, due largely to the equalization of representation in medicine between men and women, towards emphasizing quality of life in specialty selection, and this explains many of the shifts in specialty popularity.
The other change is due to financial shifts in medicine, especially wrt certain fields-- for example, pathology was largely destroyed as a competetive specialty due to hospital consolidation and subsequent massive cutting of the need for pathologists, coupled with overproduction of pathologists. I'm not sure how psychiatry salaries got annihalated, but I think its presence as a non-competive specialty is also mostly due to the very low salaries associated with it as a specialty.
Anyway, given the very short relative period of time over which the idea of medical specialties has existed, and the lack of cycling of popular fields (it's not like neurosurgery has ever not been extremely competetive, or psychiatry has gone from popular to unpopular to popular to unpopular: I think you'd be hard pressed to name more than a couple of specialties whose popularity could be characterized as cyclcial), I don't think cyclical is the right term.