Canada

Construction is heaaaavily influenced seasonally. Thats obviously largely dependent on the field of construction (ie residential, commercial, heavy construction, underground, factory, etc)

A large number of the contractors I work with either lay everyone off, or fully shut down over the winter. As soon as things start freezing, construction costs skyrocket. Daylight hours mean fewer working hours (unless you want to provide sufficient lighting, which is another expense), quarries and pits close, concrete requires winter heat and heating for the first 3 days to sufficiently cure, etc.

Additionally, construction is very boom/bust, where the rest of the economy impacts how much work is available for them. Right now, theres a huge demand. But go back and theres been two or three big slow downs in the residential construction industry in the last 15 years, which pushes people to other jobs (as mentioned by the other poster). When the economy is slow, there's less investment in infrastructure by corporations, meaning there's less demand for factory/commercial construction, and the host of trades that go with it.

Trades are a specific job that often have lots of working experience, so when a good quality tradesperson leaves, its hard to get the experience and knowledge to replace them effectively.

Wouldn't that be due to the weather?