.dmg no longer needs password

...the password's in my keychain. Why does it suddenly no longer require a password?

The answer is right there, but it seems you are unclear on what the keychain does. The purpose of a keychain is to store commonly used passwords so you don't have to type them in each time. The whole keychain is unlocked when you log in. So if the password for your secure disk image is stored in your keychain and you are logged in, your encrypted disk will mount without asking for a password, since it just gets it from the keychain.

You need to delete that item from your keychain, and when you open the .dmg file be sure the box for Remember password (add to keychain) is unchecked.

My pleasure! Thanks for the star.

Personally, I think it's unfortunate that when you create a secure disk image in Disk Utility the default for that box is to be checked - it partially defeats the purpose of the secure disk image to store the password in the keychain.

Be sure you don't forget the password. If you do, those data are unrecoverable.