This is something that happens to me also and I'm positive that it's not a sticking ALT key. Reason being that it happens after the pc has a hissy fit sometimes resulting in a "blue screen" error message. Usually some of the tray icons sitiing in bottom right corner beside the clock also disappear. Also often accompanied by the order of opened program tabs at bottom of screen being re-arranged.
After this all happens any use of the TAB key seems to flick between applications and not between fields. Quite annoying and the only way to fix it is to reboot. After which all is back to normal which is why I discount a sticking ALT key.