There's only two ways to get flagged: automatically via defence mechanisms, or submitting something HN users deem inappropriate.

There already are moderators and mechanisms for HN users to "vouch" for flagged posts too, your post doesn't seem to be flagged now so I guess some people did that.

Yes, now its not flagged. Thanks for Good people of HN who vouched my post. And, thanks to you as well for explaining it perfectly.

Your post reads as an advertisement.

That is most likely why the number necessary of users flagged it.

Flagging is from normal readers hitting the "flag" button (those with enough karma to obtain flagging rights).

I marked the post as ShowHN. And, my understanding is ShowHN is product promotion only that user have developed. Is my understanding wrong?

Regardless, reading as if it were "advertising" would possibly trigger the anti-advertising users, and HN is likely to have a slightly higher percentage of "anti-advertising" users than found in the general public. They may never notice the "ShowHN" label once they are triggered to think "just another SPAM".