While I think Bootstrap is great and I use it frequently, I feel like it's initial popularity has been in large part because it has the backing of Twitter. What I'd be curious in reading about is how relatively unknown developers are able to get their open source projects seen and known.
I think the project itself just hit a nerve. Twitter has released plenty of code in the past that hasn't become this popular. Personally, Bootstrap fills a very valuable niche in my work, and a large part of that had to do with the polish and completeness it had out of the gate (you can use it as a self-documenting style guide, e.g.), so maybe this can be ascribed to Twitter in terms of resources. Despite this, I think it's more "right time, right place" than anything else here.