This is from the official blog:
We are writing to let you know that there may have been unauthorized access to some Twitch user account information.
For your protection, we have expired passwords and stream keys and have disconnected accounts from Twitter and YouTube. As a result, you will be prompted to create a new password the next time you attempt to log into your Twitch account.
We also recommend that you change your password at any website where you use the same or a similar password. We will communicate directly with affected users with additional details.