When your generals move into enemy territory, his army becomes susceptible to a hit on their morale. The longer they are out there in enemy territory the bigger the morale hit and the more likely it is to occur. Having a general with the logistics traits and/or with good management reduces the likelihood of suffering from bad supply lines.
EDIT: Why the hell did you post this twice? There was only an hour between the first time you asked and the second. How impaitent are you?