WTF ???

That's either your graphics card dying a slow death or corrupted files.

You can download LoL again separately and see if it gets fixed. If it does, the original files are corrupted. If it doesn't and the same thing happens in other games, your video card is indeed on its last gasps. If it does for a while, then suddenly starts screwing up again, your hdd is screwed, unless you have an ssd, in which case your windows is screwed.

Trust me! I'm some random guy on the internet.

Edit: You could also update drivers, physically clean the hardware for dust, run defrag, check for errors, scan for malware, check part temperature during peak usage or just play like that and hope it goes away.