Either option is fits Harbinger's final threat from ME2:
Human, you've changed nothing. Your species has the attention of those infinitely your greater. That which you know as Reapers are your salvation through destruction.
It's probably a combination of both: the Reapers have noticed the tendency of humans to throw monkey wrenches into their plans, which has them both nervous and angry.
But... he said what Shepard did changes nothing. You know what, never mind. If you like Mass Effect focusing on how humans are so special and how Earth is the end all be all place of the galaxy, knock yourself out. Me, I enjoyed the previous two games where Earth was hardly a factor and humans were simply a part of the galaxy, not the center of it.
I have enough science fiction stories covering books, tv shows, movies and games where Earth is being invaded by aliens. I don't think Mass Effect needed to resort to that trope in order to tell a compelling story with high stakes.