Ok. Just checked myself.
If the page is downloading and displaying properly and what you are talking about is that the active/spinning wheel all the way up on the status bar, next to the signal strength, then I see it too.
Actually, it spins on pretty much every page of the site, not just the Active Topics.
There is probably an active element on the site that keeps the connection open. It could be an add, or some part of the page automatically updates without you having to manually refresh.
I've seen it on other sites as well.
Or, it may have nothing to do with the site at all. That activity indicator is for the iPad as a whole. It will spin if the iPad is doing downloads in the background as well; say for app updates.
Addendum:
I just retested this site and a few others before clicking Post Reply, and the spinning has stopped. Even for the Active Topics page. That suggests that my iPad was indeed doing something in the background. I took a look at the App Store and see I have several recent app updates, and eight more waiting to update; so I suspect that's what was/is going on.