Criteria for: Nabble forum scheduled for deletion?

[MichaelAtOz]

I have a test forum for the main forum, so it doesn't get much use
Periodically i get the email 'Nabble forum scheduled for deletion'
But I also periodically post something there to try to prevent that.
I got one today, when I clicked the link:


It shows I posted 14th May.

So what is the criteria that causes the 'Nabble forum scheduled for deletion' email?