The phrase "runaway hit" means a very successful and popular thing, like a movie, song, or product, that becomes extremely popular and widespread. Full definition
There are some standards or units of instruction that, for whatever reason, you know aren't going to be runaway hits with students. (edutopia.org)
Everyone expects blog entries about the Common Core to be popular, and a list of the Top Education Tweeters is bound to get a lot of clicks, but the unexpected runaway hit of 2015 for the Education Next blog was a post that Leslie Kan of Bellwether Education Partners wrote explaining why teachers in some states do not receive Social Security benefits. (educationnext.org)
All were filmed, edited with a bit of style, and set to music; all became runaway hits on YouTube and had the media beating a path to Block's door. (automobilemag.com)