I just ran into this issue, it turns out one of words in our moderation list was “ass”. This was causes any topics with words like class, associate, assembly, password etc to be held for moderation (pending status). After a bunch of testing the only solution was to remove “ass” from the moderation list.
I’m having the same issue, all topics created by participants get set to pending. Then a moderator needs to open the topic. It is frustrating the participants. I’m using the twentytwelve theme which has been customize. so far I’m not seeing any code setting topics to pending. I’m still digging, but if anyone has an idea where to look that would really help.