I find C, your "most likely" option to be the least likely. I believe this because I'm under the impression (possibly erroneous) that our busiest monstologists have either given up monstology (when it was discovered that the invaders reset a whole mess of block) or simply left the game over the past year or so. If the invaders are a response to monstology, their invasions would have been many times worse last year.
If I had to guess, I'd go with a combination of A and D:
D) It's a statistical outlier, uncommon but bound to happen occasionally.