Okay. Now I found out why I can't see the debris in Tracking Station. It was because I didn't turn on the visibility for debris. But I just don't know why the focus is changed.
My theory:
Somehow, the rocket became corrupted until it can't load itself, so it tries to focus to the nearest vessel. Lucky enough, that debris was the closest. Then, the game keeps the information to just load that vessel every time the rocket gets corrupted.
(Note : It's just what I think going on.)
Also, I forgot that it actually had the Periapsis of 28km and not 21km. At least the difference was not 50km...
Thankfully, that debris already has the parachute armed. So, I switched to it and thankfully, the parachute does not need any vessel control to open up. I just knew it was a Drogue parachute so the bottom engine just exploded. At least not the whole thing exploded and I recovered it.
Did you know? The debris was there for 6 kerbal years.
Adjie's post in Weird vessel focus change - help me please was marked as the answer