Skeletons in the field before the three-eyed crow's tree
In "The Children" (episode 10 of season 4 of Game of Thrones), Bran's party reaches the large Heart Tree, but in the field before, skeletons rise up out of the ground and attack them.
There does not seem to be any explanation for these skeletons. They just kind of appear out of nowhere for no apparent reason.
Is there a back story for these skeletons in the books, or is it just some random plot device?
These are skeletal wights. These are men turned into wights due to the magic of White walkers (called as Others in the books). These were guarding the cave of three eyed Raven from others to enter. They are same as a normal wight and attack warm blooded creatures. These appear to be skeletons because the white walkers reanimate unburnt corpses which are in a state of little more than skeleton. They were lying in a motionless state waiting to attack someone who reaches the cave of Three eyed Raven.
These wights near the cave were originally men of the Night watch who lost their lives as a part of their duty and later turned to wights by the magic of White Walkers. These looked like dead men with pale flesh and black skin in the books.
That was when his shout became a scream. Bran filled a fist with snow and threw it, but the wight did not so much as blink. A black hand fumbled at his face, another at his belly. Its fingers felt like iron. He’s going to pull my guts out.
But suddenly Summer was between them. Bran glimpsed skin tear like cheap cloth, heard the splintering of bone. He saw a hand and wrist rip loose, pale fingers wriggling, the sleeve faded black roughspun. Black, he thought, he’s wearing black, he was one of the Watch . Summer flung the arm aside, twisted, and sank his teeth into the dead man’s neck under the chin. When the big grey wolf wrenched free, he took most of the creature’s throat out in an explosion of pale rotten meat.
Excerpt from Chapter Bran II A Dance with the Dragons
Normal Wights in the books are described as slow and clumsy, queer . But in the show adaptation, Director Alex Graves changed dead men to skeletons as a homage to special effects legend Ray Harryhausen's famous stop-motion skeleton warriors fight scene in the 1963 film Jason and the Argonauts. He stated it in an interview to Variety:
When I read the outline, I called David and Dan, I went straight to Hollywood and met them and I said, ‘Are we talking about the zombie guys that we’ve been doing or could these guys be viciously dangerous?’ They said, ‘Oh, yeah, that would be great.’ So they go across this snow plain and skeletons start to come out of the snow, à la Ray Harryhausen, who we sort of privately dedicated the sequence to. They come out of the snow at 90 miles per hour, and they are there to kill Brandon and Jojen before they get there, and they’ve been waiting for like a thousand years. Nobody knew about the sequence and it [wasn't] in any of the marketing, which is the most brilliant marketing move I've seen.
Many more skeletal wights are seen in Season 5 episode 8 Hardhome.