What buttons do you press to use your UAV

You can press down or up on the D-Pad to cycle through your scorestreaks. Whichever one is highlighted in the scorestreak list is the one you will use next.

If you get an item via a care package, it will be in a fourth slot at the top of the list. Otherwise, they will be in the order they are unlocked.

Whichever one you've unlocked most recently is the one that will be used by default.

As Zombies mode is technically a 'cooperative campaign' mode (rather than a 'multiplayer' game), the same auto-aim assist which exists in the single-player campaign carries over into zombie games.

So what is happening is that you're turning in circles, and as your crosshair passes over a zombie it slows down your turning. It helps if you wanted to aim/shoot at that zombie, but in other scenarios (quick turning to run from/melee a closer zombie for example) it can be quite annoying.

You can disable aim assist in the settings. I don't own Black Ops 2, but if memory serves from earlier games it is under the Look Sensitivity sub-menu and is called Target Assist.

It appears Black Ops 2's Zombie mode has it's own aim assistance, separate to the option you can toggle in the settings menu (Source). Which can't be completely disabled.

This may be a bug on Treyarch's end, or by design. You could try contacting them with the relevant details, to see if this is the case.