Questions are usually protected when there is a slew of non-answers being posted as answers.
Examples include:
- Thanks!
- I have the same problem, how can I fix it?
- I'm too lazy to get rep so I write a comment, so here's an a non-answer for the mods to remove.
- I haven't read the FAQ, so here are more details for my question, posted as answer.
Usually in such cases, the question will be protected by a mod, a high rep user (15k and up). If I remember, under certain conditions, the "community" user (which is actually a bot) will also protect questions automatically.
You cannot simply trigger protection via a flag, however, as a real person must accept your request and follow through with protecting the question. However, if you believe a question should be protected I would highly suggest flagging or even leaving a comment as someone will likely find it quickly and handle the situation.