First, if CSM is enabled, you are not UEFI booting. Windows 8 UEFI booting installations still put a MBR on the OS drive, so standard MBR/BIOS booting is possible. CSM is really the main switch (currently) that determines BIOS or UEFI booting.
The current settings used by most mother board manufactures to enable UEFI booting are inconsistent, confusing, missing or at least not documented well at all. But it seems ASUS has built some checks into that UEFI to prevent problems. IMO, that is why CSM is always reset to enabled.
The issue probably is, UEFI booting requires the video device to support the GOP video protocol used with UEFI booting. 95% or more of video cards do not support GOP in their Vbios. Which means you can't use them when UEFI booting. Guess what video source supports GOP? The Intel IGP on Sandy Bridge and Ivy Bridge CPUs... I don't know which AMD CPUs with on board video support GOP.
So I'm guessing that your UEFI is detecting a non-GOP compliant video device, and changes CSM back to enabled. It could also be the Other PCI Device ROM Priority set to UEFIOpRom, which either does not exist, or is detected as non-GOP compliant. If I disable CSM when my video card is the selected video device, I get an error during POST that the video card does not support GOP.
I don't know what CPU you have, but you probably have one with an Intel IGP. You could switch to that (gasp!) to test UEFI booting. That is what I must do to get UEFI booting to work. EVGA has been providing a VBIOS update on request for their 600 series video cards, but I don't know what other video card manufactures are doing for their products. I just learned of this VBIOS update, so have not tried it yet.