* pissed off a superior by beating him in gambling or a sport.
* was assigned to care for the King's horse or hunting dogs, which just happened to be injured or got sick.
* was defeated in combat by a commoner ... who cheated, but no one knows that.
* hey ... there's only about 10,000 ways alcohol could have been involved.
* was set up by a rival ... perhaps the rival hid his sword so he could not report for duty, or really allowed the King's hunting dogs to get out (and the NPC was blamed).
* was lax in his duties ... maybe he was supposed to check the grain supplies for the horses, but was lazy and only checked most of it. Unfortunately those 3 sacks in back that he didn't check had tainted grain that made the horses sick.
* decided it was more important to attend a religious service than report for duty
* puked on a member of the royal family he was supposed to be guarding. The guy just got sick, but didn't want to leave his post. Or, maybe he had the runs and *had* to leave his post.
* removed from the service because a family member or close friend turned out to be a spy or criminal. Having the NPC close to the king was considered "too risky".
* removed from the service because his girlfriend kept showing up while he was working just to "talk" or ask for money.
* ruined a portrait of someone in the royal family by accidentally scraping his armor against it.
* participated in a flag ceremony, but hung the flag upside down.
* was late for duty, so he took a "short cut" through the king's private gardens. Everybody in the guard does it, but the NPC was unlucky and caught the prince having a romp with a serving girl. Prince had NPC reassigned somewhere far, far away.