Yes that is because you're using a Content Delivery Network (CloudFlare; not a proxy), they provide a free SSL for all accounts; it is what I use on all of my sites.
I force all HTTP traffic to HTTPs through a CloudFlare Page Rule:
If the URL matches: example.tld/*
Then the settings are:
Forwarding URL, 301 Redirect
https://www.example.tld/$1
The above forces WWW, you could also just trigger for non-HTTPS by:
If the URL matches: http://*.agardenersforum.com/*
Then the settings are:
Always Use HTTPS
If you're already using HTTPs through CF steps 1/2 are moot. For the rest you just need to access UBB.threads over HTTPs and run the rebuilder " Update Forum URL References from HTTP to HTTPS".