Have presidential powers evolved over time, or were they a result of a major event? Explain in detail.

It's a little of both; the expansion of the power of the President of the United States has been going on for a long time, but it hasn't simply been a gradual progression toward ever-greater power. Instead it's more what biologists would call punctuated equilibrium; most of the time it stays fairly constant, and then there are large, relatively sudden disruptions which shift it forward.

The main cause of such expansions of Presidential power has been wars. Abraham Lincoln greatly expanded Presidential power in the Civil War. Woodrow Wilson greatly expanded it in WW1. Franklin Roosevelt expanded it again in WW2. Lyndon Johnson expanded it in the Vietnam War.

Even when we weren't actually at war, expansion of Presidential power has usually been accompanied by some sort of crisis or panic, such as a recession or a major terrorist attack. People look to the President to save them from danger, and expand his power (particularly his military power) in order to do that.

This might not be such a big deal if we then reined in Presidential power again during calmer times; but that rarely happens. Instead we get a kind of ratcheting effect, where the President gets stronger... and stronger... and stronger... until by now the President's powers, especially over the military, are far beyond what was originally in the Constitution. Whether that is good or bad depends on whether you think the President or Congress better represents the will of the people and the interests of the nation; but it's definitely a major departure from what the Founding Fathers originally intended.