I find it curious that at the time you saw the table, the # was exactly 109 (which is the # of people registered in October).
Did you check the other dates I mentioned? It happened the same a those times. Why did it happen now? The forum isn't inactive. In fact, it's very active! In 23 days, 1310 posts have been posted, and that's more than in the whole September 2011! So, it's not related to being inactive... is it related to being very active? Or is it another thing? What happened at Nov. 15? 
I think that at that time, the forum was inactive because of the spambots!
This post is from January 2010, but it's also about spambots, and apparently inactivity because of them:
Ugh. Terry, it looks about time this forum got some mods. I'm watching this guy post thread after thread, and I see there are at least a couple others online right now too, and there's nothing anyone can do about it! 
[...]
man, I hope Terry logs in soon. This forum is completely useless! I wanted to talk about an impressive speedrun I saw on youtube. 
That means that if there were a lot of spam posts at the dates you mentioned, people couldn't really use the forum because of the spam. That could explain the inactivity, because of course the spam threads got deleted, but there were less normal posts than usual.
Also, when I made the screenshot of the table, the number was 109. Now it's 115 (also a coincidence, that was the total number of new members from September)
And about the users online: yes, it is related, I also noticed that often there are 15 users online, instead of less than 5. The spambots really log in to look like real users, and to get attention.