I run a program called Pop Peeper:
www.Esumsoft .com/
This is an email notifier. When it receives new messages, it pops up balloon tips, or what are called toasts now I guess.
Now, however, on windows 10, if you restart Pop Peeper, the notifications it sends out are repeated by NVDA. If you restart the program again, the notifications are repeated three times.
The only way to make this stop is by restarting NVDA. I talked to the developer of this program and he can't repeat this with narrator. I don't know of any other app which regularly sends out balloon tips/toasts in order to test the theory that it is this mechanism which causes the behavior.