What happened?

Following several hollers upstairs by me this morning there was still no sign of life from Son 1 (by 2 minutes)'s bedroom so I ventured in and was faced with this scene of devastation. To be honest, it doesn't look half as bad in this picture as it does in real life, but you get the general idea. He's an otherwise wonderful child in every way but for some reason needs to live in squalor. I try to avoid going anywhere near his room unless I absolutely have to and instead nag him daily to sort it out, which he does, but then it magically returns to chaos overnight. It's got so bad that Son 2's also started nagging him to tidy it now - and even tidied it for him the other day!