If you want to open your inventory, you hit tab.
If an enemy is kicking your ass, you may want to run, and thus you hit alt to sprint away to safety.
If an enemy is kicking your ass, you may want to drink a potion to recover some health, and thus you hit tab to open the menu.
If you hit Alt + Tab because you were running for cover and really needed to open your menu to drink a potion or whatever, the game closes because you've just done the windows command to switch to another opened program. I have done this 6 times now. It's gotten me killed three. The first time I thought I was being stupid and it was my mistake, the second time I realized lolno, I was trying to survive.
For the love of God Bethesda, at least PRETEND you put some thought into the PC controls. I know I can switch the key setup, but I do think the fact that such a combination of keys is VERY likely to happen in gameplay closes the game and NO ONE AT BETHESDA FREAKING NOTICED proves there was little to no testing done on the PC, and does suggest that yeah, PC users deserve a freaking patch for better UI
PLEASE patch the damn game and at least fix our Y-axis and the Horse controls. Fix the menu so that we can open it, access the magic menu, then access our weapon menu without having to reopen the menu. Fix the controls so that it properly recognizes my mouse clicks, and a click isn't prone to randomly closing out of the entire conversation or transaction, or so that my character doesn't randomly freeze up and become non-responsive to my clicks telling him to swing the damn claymore.