Uncapped framerate in certain screens causes high levels of GPU usage.

Minecraft's GUI screens (with no 3D rendering in the background) don't have a cap on the framerate they run at. This means that they will use up a lot more GPU power than needed.

Here's some example framerates taken from 13w10a (so I could test the texture pack screen in-game).

Title screen: 36 FPS
World List: 780 FPS
Server List: 725 FPS
Options (From Title): 785 FPS
Language (From Title): 710 FPS
Texture Packs (From Title): 785 FPS
In Game: ~95 FPS
Options (In Game): ~95 FPS
Texture Packs (In Game): ~95 FPS

Of interest is that the texture pack screen has the same forced background depending on where you access it from, but it only displays the abnormally-high framerate if you access it from the title menu (or the options screen in the snapshot).

So it's a bit more taxing for the GPU to sit at the world select screen than it is to actually play the game.

Crash report 1 (02.35.03) is from 1.4.7.
Crash report 2 (02.38.24) is from 13w10a.