Or just decided to do some non-linear storytelling, why not?
non-linear would still be chronological/causal (just that time speeds up and/or own throughout the story), be that if it were polynomic, exponential, sinusoidal, or otherwise.
To jump back and forth in time (non-chronological/non-causal) you would need the storytelling to be - while still using your preferred geometry-terminology - uuuh... multi-valued (no, that would be more about the functions that describe the shape)? discontinuous (no, from their own perceived time they are continuous...)? Concave (in terms of concave polygons)? That seems most apt, let's go with concave!
As for why not, the most general answer would be that non-chronological storytelling is near-universally bad (the exceptions being: if it
is still chronological to the character you follow (and thus also to the reader), but not to others, because doctor who is a time-traveler. or if the story is multiple mostly isolated stories so jumping around does not harm the flow too much, because doctor who/stargate is episodic and going non chronologically could even be fun for teasing future episodes).