In my experience seat assignments rarely get changed on AC. When they do they tend to be because:
1. change of aircraft
2. you reserved a bulkhead and got usurped by a family with an infant
3. a family member or the flight attendant politely asks you to move so a family can sit together
4. They issue two boarding passes for the same seat. (yes it has happened to me a few times)

5. Some prick of an SE

wants your seat and convinces the concierge to make the change.