Pinterest updated its Android app a couple of months ago and added the push notifications feature. It sends notifications for many events. To manage notifications for Pinterest Android app, do as follows:
1. Open Pinterest on your Android device
2. Tap the profile icon on the top right
3. Tap the Gear icon to open the settings
4. Under General Settings, open "Push Notifications"
5. Now mark the events you would like to be notified when happened
* By default all the events are marked.
To compelety disable push notifications for Pinterest, read this Android tip: How to disable Push Notifications on Android