> How or why did this thread title get changed / Hijacked for Battle Squadron?
> Is it a bug in the forum?

Replying to a thread gives the opportunity to change the subject of the new comment. The pre-entered subject is copied from the comment that is being replied to. This makes sense for sub-discussions in threaded view (which was removed some years ago). So yes, I consider it a bug that the subject of the most recent comment instead of that of the opening post is what's displayed as thread subject.
Replying to your comment would normally retain (or rather reinstate after Solo_Kazuki's change back in the preceding comment) the "Battle Squadron" subject of your comment as thread subject, but I changed it manually to the "Tower 57" subject.