Why?

I have said some of this before but don't know where so let's go through it again. In "Delenda Est," Manse Everard says:

"'Don't ask me why...this is the first time we came back from the far past to find a changed future. I don't understand the mutable-time paradoxes. We just did, that's all.'" (Time Patrol, p. 187)

The character saying that he does not understand signals to the reader that there will be no further explanation. The real reason is that Poul Anderson had decided to write a story in which history had been changed but can we find an explanation that will make sense within the story?

Let us imagine the following sequence of events:

Everard travels from 1955 to the Oligocene and returns;
this round trip has occurred within the Time Patrol timeline;
after Everard's return, a Neldorian (who had maybe been stranded in the twentieth century) steals the timecycle that Everard had used;
this Neldorian and his companion travel to the Punic War period and help Hannibal to sack Rome, thus generating a divergent timeline;
Everard, using a different timecycle, travels from 1960 to the Pleistocene and returns;
this "time," Everard sets out from the 1960 of the Time Patrol timeline but returns to the 1960 of the divergent timeline.

Thus, there is a causal sequence from "Everard travels from 1955..." to "...returns to the 1960 of the divergent timeline." I have invented a simple example. However, such causal sequences, beginning in earlier timelines, might become so complicated that no one would be able to trace them back.

The same question is asked again in The Shield of Time:

"'Apparently only a relative few entered [the divergent timeline], including you. Why not many?'" (The Shield Of Time, p. 304)

This time, Everard gives an answer that does not work, then again admits to not understanding:

"'Those were just the ones who happened to cross the crucial moment, bound uptime, in that larger section of time during which there were related events, like the Patrol's salvage work. We've got a longer section now, with a lot more traffic in it, so our problem is correspondingly bigger. I hope you understand what I'm saying. I don't.'" (ibid.)

Do time travelers in the Time Patrol universe "cross moments," like Wells' Time Traveler or Anderson's Jack Havig? Time Patrollers seem instead to teleport across time, i.e., to disappear at one set of spatio-temporal coordinates and to appear at another without having crossed the interval between. But if they do cross moments and if "the crucial moment" is the moment of the temporal change, then this moment is crossed by every time traveler returning uptime from far enough back. And they all pass through the "section of time" containing related events.

When Everard admits to not understanding, Unattached Agent Komozino responds:

"'It requires a metalanguage and a metalogic accessible to few intellects...We haven't time to quibble about theory.'" (ibid.)

In other words, we have pursued the discussion a little bit further than in "Delenda Est" but now let's get back to the story!