Yeah it's most likely related to any google search you have done, whether through a computer or a phone that was linked to your google account. I say this because Google Now has given me information about recent video game titles I have searched about, such as it being reviewed, or a new blog post by some random person about it, etc. Then, if you click the top right overflow menu button on that specific card, it asks me if I want to continue being notified about that particular game.
Knowing this, I then searched for some of my favorite artists and actors/actresses. When the card popped up, it asked me if I'd like to be notified before the artists' new album comes out, or the actor's/actress's next movie comes out. I LOVE this!

It even works on obscure people (I say obscure to mean non-U.S related; I have Google Now notifying me of my favorite Japanese artists and actors).
Google Now may be somewhat creepy sometimes, but damn I am loving it. Creep on, Google, creep on.