Have a look at the posts revisions. What you can see here is an edit war - the OP clearly took exception to the community response (i.e. downvotes, various comments calling into question the scope of the question) and added a rather large section to the post to let off steam.
This was removed by community members and the OP repeatedly rolled back the post to include said content. To prevent this, Shog9 locked the post, which prevents anyone but a diamond (a site moderator or stack exchange employee "carrying" a diamond - not all do) doing anything meaningful to the question at all, including vote, edit, close, delete, undelete, answer etc.
Since users have most of these permissions on their own posts, locking is a last resort option which prevents even the OP from altering their own content. Stack Exchange content is contributed CC-by-SA, but once contributed you cannot revoke that license to your content and vandalising it or altering it in a way that is not constructive in the community needs to be prevented. That is what has happened here.
The idea is to force such discussion to resolve itself on meta whilst not allowing the post to suffer in the dispute. The OP is, therefore, welcome to wander over here and ask why their question was locked and hopefully achieve community consensus on what should be done. Clearly that discussion hasn't started yet; in this case, I think it would require the OP (or another person?) to ask why his question was, in the eyes of some, considered too broad.
As to why Shog9 stepped in - I don't know. Perhaps he saw the issue first? Perhaps he was checking up on our moderators/this community in general (that is his job, after all) and just saw a flag? Who knows; I don't. There is, however, nothing wrong with what he has done. He hasn't taken any action our moderators cannot undo; he has, however, prevented a problem from escalating. Shog9 has a lot of experience with this sort of stuff; he was hired precisely because of his contributions to SE and experienced a time before tools such as locking existed. I don't see a problem with SE staff helping out where their speedy action stops a problem dead in its tracks and encourages a community consensus, rather than the abuse of a post, or what you might see elsewhere as a derailed thread. Moderators are not all-present users and whilst there are three of them, it can happen that all three moderators are away for a time, particularly at weekends.