That's because your IP address has changed and SE are not sure whether your account has been hacked or not.

The only way to avoid having to reset your password every time your IP address is changed (such as playing from a different location or if your internet provider assigns you a dynamic IP address) is to purchase a Square Enix Security Token or Software Token and enter the random one-time password it generates each time you login (doing so causes the system to ignore changed IP addresses). It's a security feature to protect your account.