A real simple question
The first time I can remember studying the Great Depression in school, it was probably in 7th or 8th grade history, the simple answer to "What was the trigger that caused the Great Depression" was: An international trade war set off by the Smoot-Hawley (both republicans) Tariff Act.
The world economy is past time for a recession, and Trump's action today on steel and aluminum imports (which has already caused the market to drop over 500 points) could be the spark to set it off. I know a lot of people, especially those on the low end of the income scale, get hurt during recessions, but if the blame on Trump causes him and the GOP to wiped out in the next two election cycles it will be somewhat worth it.
(Most economists agree that buying stock on margin and other problems with the stock exchanges were the underlying cause of the Stock Market crash that led to the Depression, but the tariff bill was what caused the system to come unravelled when it did.)