1) I started it because it was simply a personal CSS framework I was using myself to kick start my projects, and I wanted to share it around.
2) People found it through various websites, organically I guess.
3) I initially only posted it here on Hacker News I think. Then it was trending on Github, and other people posted it on Reddit, and other websites like Codrops.
4) I've never really tried to market Bulma. It's just a solution to a problem I had, and it also happens to solve a lot of other developers problems.
I think if your open source project solves a common issue, people will eventually find it. I discover lots of valuable tools when trying to code something.
If you need a strategy, just copy what a successful startup would do: solve a problem that lots of people have. An open source project just happens to solve a problem you personally have, and you assume others will to, so you share that knowledge.