Background, Exile

Not every crime recieves serious punishment. Some minor offences may result in a fine or time in the stockades. You have to really go out of your way to commit a crime that would get you exiled, but sadly that's exactly what you did.
The who, what, and where are all up to you. Here are some broad stokes concepts on why you might have been exiled:

Serious Crime

You commited a truly serious crime, or at least you were found guilty of one. Maybe you are innoncent, maybe you are guilty, either way this was no petty larceny. Murder is the most serious offense. Normally that would get you executed, but perhaps there is a reason to exile you instead. Perhaps you are the son of a nobleman who has requested mercy and it has been granted. Perhaps there was a bribe and money changed hands.

Minor Crime, Repeat Offender

Perhaps it is not the gravity of one offense, but rather the proponderence of many. Perhaps you stole the local baron's horse and this is the last straw, if you will pardon the pun. You might have been a more petty criminal, but the fact that you have found yourself on the wrong side of the law one too many times has led to you being banished.

Political Reasons

You were on the wrong side of history, or so current rulers of Aeritain think. Perhaps you were part of a group who did not trust Lord Athenry and wanted a different regant or for Prince Edwin to rule immedielty somehow. Whatever the case, you opposed the ruling regime's policies, expressing dissent or participating in activities deemed rebellious.

Religious Heresy

The Covenant is ubiquitous in Human kingdoms, and Aeritain is no exception. They wield an exceptional amount of power and more than a little political sway throughout the realm. Something that you have done has seen you accused of heretical beliefs or pratices and had led to you being banished from the land.

Accidental Tragedy

Some tragedy has occurred. Liekly it wasn't your fault, but you are being held responsible all the same. Those holding you to account may feel sympathy for you, perhaps that is why you are being exiled rather than executed. It's probably a small consolation.

Forbidden Romance

You engaged in a forbidden romantic relationship, going against societal norms and resulting in exile. Perhaps you were having an affair with a noble's wife, or deflowered his daughter. Whatever the case, you are paying the price. Was it worth it? Only you know.

Failed Duty

You failed in a crucial duty assigned by the ruling authorities, leading to severe consequences. Perhaps you were a bodyguard for the local lord, but fell asleep and thus failed to stop his assasination. Whatever your failure, it has led to exile.

These are just some ideas, not a comprehensive list. Use your imagination and come up with something that works for the kind of character that you want to play.