I don't know every reason, there are too many of them. However, I know two and can tell about one. A while ago I got promoted when my direct senior was moved to a different, more challenging role at the same company, and when CEO decided I'm good enough to do my direct senior's job. What was interesting about it is the fact, that there was another guy who was working at the company in the same role for 2 years more than me. Yet, I got promoted, and I know why. This guy was constantly causing troubles and abused his duties. So when it was time for changes in the company, I believe CEO thought of me as a more reliable employee. The moral is: do your job well, don't abuse it, become better and seek for changes - they are the opportunities for promotion.