Why the sudden popularity? It's not just the number of jobs the economy has been adding over the past few months. It's a triangulation of three trends that are particularly impacting this type of application.
I assume the term killer app came from the phrase "to make a killing." I never thought much about the metaphor -- the use of the word "killing" as a synonym for "succeeding" or "winning." But it's a little creepy to think about what the word killer really means. After all, a "murder app" or "homicidal app" doesn't sound appealing, and yet all these words are interchangeable synonyms.