YouTube has recently started to experiment with replacing Google+ notifications in the navigation bar with YouTube notifications. You get notifications for recently uploaded videos from your subscribed channels, but only if you’ve enabled notifications for those channels. For example, you can go to the subscription manager and click the bell icon next to a channel to enable or disable notifications.
The settings button sends you to the Notifications section from YouTube’s Settings page and the 3-dot icon next to each notification lets you turn off notifications from the corresponding channel.
If you don’t like this experiment, you can always clear cookies for youtube.com in your browser’s settings and opt out.