In the game of Sburb, whenever there is like, a scratch, the players get replaced by their ancestors or guardians, and the players become the ancestors or guardians. Why is this?

While this is what we see happening in both Troll and Human Sessions, this isn’t necessarily what needs to happen! Skaia is a well of Knowledge, as Rose explains in Act 6 Intermission 2.

http://www.mspaintadventures.com/?s=6&p=006606

ROSE: To understand what happened, it really helps to understand exactly what a scratch is.
ROSE: When John severely damaged the Beat Mesa on your planet, and sent it off to Skaia to release its temporal energy there, you could view it as a kind of “request.”
ROSE: We were asking Skaia to change everything at a fundamental level, and we gave it the energy to do so.
ROSE: But Skaia is a very passive entity. It only “knows” and “sees,” but it never quite “acts.”
ROSE: When it is asked to change everything, there is only so much it has control over.
ROSE: In fact, it has control over exactly one thing. The defense portals.
ROSE: It can decide to send important meteors to different points in time than originally planned, thus creating alternate realities.
ROSE: Offshoots of promise, rather than futility.
ROSE: And it turns out the most important meteors of all tend to be the ones delivering the young players to their planet.
ROSE: So all it has to do to change everything is tweak their destination times a bit.
ROSE: All internally-prompted changes in the post-scratch universe are decided entirely by this modest adjustment to the parameters.
ROSE: It’s a very simple concept, actually.
ROSE: Yet the consequences are dramatic. It results in not only a hard reset for the session, but a partial reset for the universe too, due to the many causal entanglements between a session and its originating universe.

When a Scratch happens, Skaia shifts around the time of arrival of the Meteors, which alters the time period every one of them is sent to. It makes sense that the ones that were sent Before will now be sent Afterwards and viceversa, but there’s no written rule that says a Scratch necessarily needs to switch Guardians with Descendants, and I could definitely see a weird Fan Session where Guardians and Descendants are mixed together in some manner. Like imagine the Lalondes and Striders being Players, with John, Jade, Jake and Jane being the Guardians, that’d be intriguing!

Of course, as we know, this switching system has a glitch, that we see in action in the Alpha Troll Session. Only the Meteors with the Kids switch the Time they’re sent to in this case, that is to say, the only thing that produces this new Reality is when the babies are sent. And since the babies are sent at different times, a different group of Players grows up to Play SBURB, effectively playing a Session that’s different than the original. Because of this, the only Meteors that are sent to Earth in the Alphaverse are the Baby ones- There was never a Reckoning within their Session, and so the only two Meteors that reached Earth from the Alpha Session were the Frog Temple, which needs to exist to even begin the game, and the Meteor from the Troll Session that ended up being home to Calliope and Caliborn on Earth-C. All 8 Meteors to ensure their Birth were sent from the Session that ceased existing the moment the Scratch was performed.

Because of this System, where Alternate Realities can be formed where only SOME of the Meteors Land, the possibility of a Universe being Glitched, and thus incapable of producing the Babies through Ectobiology, is there. It’s implied to be a Glitch influenced by Lord English himself to bring forth his existence, but it’s never fully explained why it happens. We only see its consequences, and that is a Session with Ectobiologically Created Players that didn’t create Themselves, and thus NEED to perform the Scratch to retroactively exist. The Timeline in this case demands for the ‘petition’ to be made to Skaia and the Universe to be reset, because otherwise the Players wouldn’t have been born in the first place, and this inconsistency cannot be allowed in Paradox Space.

And that’s how a Scratch’s Baby-Timeline Fuckery works.