The thirteenth Lord Commander of the Night's Watch found in the Haunted Forest a cold woman with bright blue eyes, seemingly a female White Walker. He brough her back through the Wall and declared himself "Night's King" and her his Queen. For thirteen years they ruled over his brothers in the Night's Watch, making human sacrifices.
In the TV series, the Night's King reanimates dead men into wights with only his will, and turns Craster's sons into White Walkers with his touch, abilities that have not been specified in the books. In fact, the fate of Craster's sons is still unknown: Old Nan's tales allude to the Others' feeding human children to the wights, or that wildling women lay with White Walkers to make horrible half-Walker babies, etc. Craster's wives do say that they believe that the infant sons that Craster gave the Others as sacrifices were turned into new White Walkers - but it wasn't clear if this is what actually happens, or if it was just the wild suspicion of Craster's frightened, isolated wives. When Samwell Tarly is told to flee with Gilly and her newborn son, Gilly urges that if he doesn't "they" will come for him. When he asks who "they" are, another wife says: "The boy's brothers...Craster's sons. The white cold's rising out there, crow. I can feel it in my bones. These poor old bones don't lie. They'll be here soon, the sons." Confirmation of what exactly happens to Craster's sons has not occurred in the books yet.
This why the theory posted in this reddit link is so intriguing to me (props to VAiz4hustlaz for finding this).
There's just enough lore and legends that tell of a time where the White Walkers and Humans at least were able to interact with each other even if it's on very specific terms.
So the question is what set them off now, why are they attacking humans NOW?