Simple fact of human nature. People cannot handle great, instant successes. That's because it's not wire in our biophysiology, as there were no great successes for the first 5 million years of our species's existence.
So, a leader can feel invincible after just a couple big wins. That's when you know you're right and your advisors are wrong. Or that God is on your side and it's OK to torture.
On a smaller scale, when you get 10 employees under you, you instantly forget about their humanity and expect them to do the job until it's done, with or without breaks.
Some people in some families and in some cultures value shared responsibility and success. In those cases, a leader rarely becomes a tryant.