It only takes one thing to happen to make something go viral. It is a matter of luck. Someone out there pushed this and someone else pushed it and suddenly everybody was talking about it. At least I assume that is how it happens. Since I have yet to go viral I am still working on that. But, it got popular because of a few people that kept pushing for others to spread the word.
On a different note - I am on the border of whether or not to call this a decent book. The writing wasn't horrible, but it wasn't great either. The characters weren't really even relateable. But, if you can get past the 1000 pages of sex scenes, it wasn't too bad of a plot. And to top it off, because of the simplicity of writing, it is actually a fairly easy read. Sometimes people write too complicated which for me is an immediate turn-off.
But for the 50 Shades of Grey - I think the popularity has more to do with luck than anything else. Oh - and knowing that it was Twilight fan fiction probably helped some to. If anything - it had Twilight fan's interest piqued!