World Lines Tightening

In Poul Anderson's "Star Of The Sea," Time Patrol agent Janne Floris disguised as the goddess Niaerdh tells the sibyl Veleda to stop preaching war and to start preaching peace with Rome. Veleda cannot do this until her companion Heidhin has been released from his oath never to make peace with Rome. Floris proposes to take Veleda to Heidhin on her timecycle which Veleda perceives as "...the bull of Frae, cast in iron..." (Time Patrol, p. 611).

Everard, communicating subvocally, warns Floris that the rebel leader Burhmund is currently visiting Heidhin and offers to find another time for Floris to approach Heidhin but Floris replies:

"'No, wait. This may be a stroke of luck.' Or the world lines tightening as they seek to regain their proper configuration?" (p. 615)

Floris plans to whisk Heidhin away and hopefully persuade him so that "'...Heidhin the implacable suddenly converted...'" (ibid.) will make a suitably strong impression on Burhmund. What does "...the world lines tightening..." mean? It seems that, if a time traveler changes a past event, there can be three consequences:

(i) it does not matter because that event is too insignificant to affect history;
(ii) the event was a nexus so history is changed;
(iii) the event has the potential to change history but other events change to compensate - the world lines tightening.

When Everard tries to prevent a divergent timeline by appearing as an angel to Lorenzo de Conti, the latter grabs Everard, realizes that he is human and fights. After killing Lorenzo, Everard remarks:

"'...the tide was carrying him...trying to preserve its twisted future - Let's hope we've broken the spell at last.'" (The Shield Of Time, p. 428)

A timeline, like an organism, protects itself if possible.