Ever start a long-running process, like a build or something, and then switch to another window, to, ummm read some documentation, and then forget about it for a long time?
This will trigger a notification if a process that you've left running in a vscode terminal completes.
If you don't want it to run in a certain workspace, you can disable the extension in that workspace.
If there's popular demand, I'd consider adding some configuration like "don't notify for commands matching this regex" or something.