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.