Are you subscribed to any channels? The only other thing I can think is that you've been using this account without subscribing to channels and YouTube is offering the suggestion.
This blue pop up is exactly the same when you first setup other google properties like Adsense, AdWords, analytics, etc. They show blue pop ups like that to teach you how to use the service. So, I assumed your account must have been new.
I don't know of any way for a channel to initiate a pop up for viewers to see. This had to be driven by google's assumptions about how you use youtube, trying to offer "help".