It is due to the 'Hot Network Questions' effect. StackExchange uses an algorithm to search all the sites for questions which are 'hot'. I don't know the details of how they decide, but I've noticed that questions which receive multiple answers very shortly after posting often make it to the list.
Once a question is considered 'hot', it shows up in the list which is on the lower right-hand side of every SE site. This shows the question to an audience which is many orders of magnitude larger than the one that frequents this site. If the question is truly interesting to a broad audience, then the effect gets amplified, and the question can stay on the list for many days.