1. It should be the same things that cause the PC to fall unconscious:
- Injuries
- Fatigue
- Blood loss
- Shock (I had one character that tended to faint when injured. The last time was a bite to the hand by a wolf, something the character never woke up from, presumably because the wolf wouldn't stop the assault at that point).
The two first factors add up in some manner, but while you're rendered immobile at 100% penalty, you don't fall unconscious at that threshold.
Blood loss also factors in together with the other factors in some manner (a character can faint due to blood loss but wake up still bleeding).
I don't know if head trauma provides an increased risk of unconsciousness.
2. Weapon skill affects the outcome of weapon attacks, so a higher skill should increase the chance of an attack resulting in a more serious injury. Attacking an unconscious creature with a melee attack results in a guaranteed hit, and the target doesn't exactly have a wealth of factors to reduce the damage roll.