This is all from memory of similar incidents, so it may not be 100% accurate:
Forbidden is what happens when either the member has blocked you, or blocked new followers in general. You can only join the later if requested, or if they follow you first.
It also happens if that member has been banned. If you are trying to follow people who have randomly tweeted to your account, this is the most likely cause. Lots of spammers out there, and they get canned pretty quickly.
I'm one of the reasons they get banned so quickly. I block (if unsure) or report spammers immediately rather than just ignore them. I find the practice annoying. Both actions trigger a review of the account. A report gets higher priority.
A spam is most offen a "see this" sort of thing, or a short (banal) saying followed by a shortened URL.