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.
Yes he did. By which point the Collectors had already been focusing on humans for a year and a half.
Why are you so hung up on the human-centered emphasis of Mass Effect 3 when we already started down that road in ME2?