This is due to what we call "The RUn-aways"; there's a few threads on this already in various forums, but it boils down to this: When you press shift + Movement keys, or ANY key + movement keys, even if your movement keys are blacklisted/do-not-pass, it still passes them, but does not recognize you "letting go" of the key, and sends the alts running on that key/those keys indefinently until you press them again.

2 fixes: Hold shift/alt/whatever key you pressed and spam all 4 movement keys + strafe/turns, or add shift/alt/ctrl + movemnet keys and movement key + movement keys to your do not pass/blacklist.

No fix yet but I'm sure Keyclone's working on one if it's possible to be fixed.