What steps will reproduce the problem?
1. Ensure all WoW settings are set so that none of your attacks will auto-target a hostile target.
2. Enable Bartender4 addon.
3. Hit any key bound to a Bartender4 slot which has an attack in it.  It will auto-target the nearest hostile.

This is an old bug.  It was reported back in 2008 and I have no idea if it was ever fixed.

This bug is tied to Bartender4 alone.  Once I have disabled Bartender4 and use regular Blizzard bars, auto-target does not happen.
Re-enable Bartender4, hit attack key with no target, auto-target happens.

This is bad when you're spamming keys in your rotation and you end up chain-pulling more mobs because of this bug.