Posts by new users may in some cases be checked to make sure they aren't from spambots. We're sorry for the brief delay it can produce, but it helps us keep spam out of the forums .
It can happen anywhere on the forum when a post is made by a new member. It depends on a variety of things, which we're not going to divulge because that would - at least in part - defeat the purpose.
Like I say, we're sorry for the delay it can cause in some cases, but the overall advantages for the forum at large outweigh the temporary (for the individual member) inconvenience.
We had a temporary issue where a greater-than-intended number posts were being accidentally flagged and held for moderation review. The issue has been resolved, so very few legitimate users should now find themselves running into it.