Both are true: things build for a long time but can escalate quickly. Trump's rise to power has been born on the back of a large number of simmering, slow-developing forces.

But look at how fast things can go. The establishment of the Nazi party took a long time to boil up, but look at how quickly things developed in 1933. Within a month, Hitler is appointed into a new position, and then the Reichstag Fire Decree suspends civil liberties. A month later, the Enabling Act hands unlimited power to Hitler. And look what quickly rolls out from there: book burnings, mass executions, etc.

Things may have long histories and take a long time to build up, and we can take time to trace them back, but that doesn't mean that they don't unleash nearly-overnight changes in short periods of time.