Apple ID Password Breach

I had a notification generated from the Passwords section in Settings on my iPhone yesterday saying that my Apple ID password had been potentially breached in a data leak. My understanding was that Apple systems were very secure and this has unnerved me somewhat. My question is how might my Apple ID password have been breached? Aren't Apple the only company to store such information? A breach of this nature would surly have been newsworthy in some way. Any light on the matter would be appreciated!

Ps. I have of course reset my Apple ID password.