Depends on what you mean by alarmist. Generally an alarmist is anyone who warns someone of something they don't believe in. The critical point of an alarmist is that what he's warning about is really not a danger, or doesn't exist.
If you're talking about an alarmist who KNOWS he's just making people anxious for no reason, then I gotta say the "hidden" agenda is pretty obvious: to cause an unwarranted ruckus.
However, if you're talking about someone who doesn't know that the danger they are warning of isn't real, then they have no "hidden" agenda. They think they're right. It is a genuine desire to help others.
I think MOST people fall into the latter category. Whether they are right or wrong, they honestly want to help people and think they should be warned. It takes a special type of sleazy (and yes... the internet is full of sleazy people like that) to cause trouble just for the sake of causing trouble.... I think we call those people "Trolls" :} and I don't think their agenda stops at being an internet alarmist.