You almist certainly clicked on the Demo, or were directed there from a websearch.

The demo simulates you logging in as "Joe Demo" and includes series of dummy accounts and transfers.

If you are absolutely positive this wasn't Joe Demo's info you were viewing, then your system is likely infected with malware, resulting in you being redirected to a false chase.com intending to capture your login credentials.

Once you "log in" on their false site, they show you a fake account. When you navigated away, you loaded the real chase site and logged in as usual.

Chase and most of my other banks use multi-factor authentication, meaning in addition to requiring your credentials, you also have to register each new device. If your online account was compromised, you should check your email. It was probably compromised as well, and they used an identification code that was sent to it to gain access to your online account.