It could be for a couple of reasons...
In December 2017 we moved to a new login and user management system. This required all users to reset their password.
At the same time we also upgraded our password policy, requiring you to have a more secure password than before.
Our security team maintains a continuously-updated database of breached usernames and passwords from hacks on databases released into the public domain.
To keep your account secure, any matches are denied and blocked in real-time.
You will be instantly alerted to the attempt, forcing you to change your password immediately.