Both UBB.threads and UBB.classic check HTTP_REFERER in a few places.
I don't think there's anything sinister about this. I've done it myself in scripts I've written. It can be useful in helping the script keep track of which page a user came from, so the script knows what action to perform. It can also be useful in making it a little harder for people to hack form-based scripts by spoofing forms.
Some firewalls, proxies, and browsers allow blocking of HTTP_REFERER output, which could confuse the script.
