A possible contributing factor just popped into my head, and only applies to a small subset of the tech industry. I know from experience that big tech companies like Microsoft, Amazon, and undoubtedly many others, all have significant numbers of workers who are non-exempt contractors(i.e. not direct employees). I don't know how much labor laws vary from state to state, but in WA they are legally bared from employing these special contractors for longer than 11 months. After 11 months they have to convert them to direct employees. So in order to shirk the law and evade paying equal benefits, salaries, options, and corporate taxes, these companies dismiss those contractors for 30 days, then re-hire them. At least in Washington state, this is a standard practice for the past decade or so. There are many loop holes and exceptions, so I'm not even sure of the extent to which this contractor law is applicable.

At Microsoft or Amazon, at least a few years ago when I saw the HR numbers, at any given time, ~20% of the total pool of workers can be temp contractors, so that is a sizable numbers of workers coming and going each December, at least on paper, even if those workers stay on as contractors for many years. (As an aside, I've always wondered why this doesn't generate the same amount of bad PR as the entire video game industry, especially the "EA slave-ship").

I don't have nearly as much direct experience outside of the software industry, but I imagine other industries are doing likewise and on a far broader scale.

Extending this employment pattern to the whole economy, it might explain a big percentage of these mysterious to pin-down "seasonal workers." Or perhaps the labor statistics are just being fudged across the board(thanks US BLS!), from the top down, to try to keep a lid on the worsening unemployment and underemployment situation. I suspect it's a lot of both.

Since the 70's America has been hemorrhaging "real jobs"—i.e. non-service-sector jobs that actually pay an inflation adjusted living wage. Those of us fortunate enough to win the career lottery by going into tech have been quite shielded from the backdrop of the economic decline, and the mass exporting of (manufacturing mostly?) jobs to overseas.