Certain events, both macro and microscopic in scope, have the potential to sever one or more timelines (the latter in the case of a nexus.)

It might help to visualize the timelines as a net thrown over the universe, its mesh distorted by the mass of solar systems impinging upon it. This image is entirely false, wrong in every way but it does help to create an illusion one can comprehend.

The timelines are the strands or threads of the net but they do not exist in the same position in space time, they are separated unaware of each other but with the potential to interact where their lines cross. This illusory net isn't a single layer but an infinite stack and their threads meander through space time in a variety of directions and in 3 dimensions.

However, by and large any single timeline might be represented by a colored ribbon which marks the progress of a species assuming it only remains upon the planet of its origin and is never visited by aliens because when this occurs the timeline becomes at risk of distortion or in the worst case, disruption.

A variety of automatic processes largely are responsible for monitoring the timelines and their response in such cases go unnoticed unless the outcome is unsatisfactory.
Charles' experiment had sent a tachyon bubble off planet whose short lived instability had caused the automated system to rescue the boy, as it perceived it and then place him in stasis whilst informing the Masters of its action.

At various times in Earths history the primitive inhabitants had been visited out of curiosity by several almost equally immature species who had only recently discovered interstellar travel. This had given rise to a legend of such visitations caused partly by careless or non existent space craft shielding and the inevitable tales of anal probes and worse told by those that had (or had not) experienced these abuses at first hand.

The Mediation Corps had quickly realized that blatant exaggeration was their only recourse in preventing humanity from taking the notion of off planet visitors seriously. So it was they who had encouraged the notion of bizarre human alien sexual relations and even helped them develop chemical pyrotechnics as previous experience with other species had taught that this was the best way to discourage further inventiveness.

There's no better way to discourage real ingenuity than to allow a species to bask in the smug self satisfaction of "Oh yes we know all about that!" and as long as all they knew was low explosives, the chance of that leading them to ballistic missiles seemed comfortably far off, at the time of the Chinese civilization.