I will attribute to a few factors , not all apply to everyonea) Song was sung by Dhanush - All wanted to see how the actor sings. Imagine how it would be if we had a leaked video of "Kya Bolti Tu" from Aamir and "Apun Bola" from SRK in those times - They would have also gone viral. It just happened that the social media is more prevalent now. I would also compare it to something like "Goli Maar Bheje Mein" ... the same reason why that became popular, it had a "something new" feel to itb) The leaked video is not from the movie - But it is actually a "Making of Kolaveri Di" - So it feels good to be shown while it is still taking shape - I am hearing that this is not the exact one (the song itself) which will be played in the moviec) Lyrics are in broken english ... we have had comedy scenes & a few funny english words being thrown in before - but almost 80-90% of the song in broken English is a USP - And it is a bit funny... though the mood of song is supposed to be serious - but the way it is sung, it does make people smiled) Again, the broken English meant it was understood by a wider audience, not just Tamil Nadu .. In fact we have version remakes being made by people from different countries , not only rest of Indiae) Now, coming to the song itself, it is not the best song ever written or anything, but it is a decent number to be hummed by manyf) "Kola veri" - As I mentioned earlier in this post is a common enough phrase, used regularly in speech to indicate one is angry/frustrated etc...locally & that being used in the song.... people felt in sync with lot of things in daily life...