This is a device for accomplishing the most slick and slippery zippery.
If you've ever looked at a toothed zipper you can puzzle out roughly how it works. Maybe not exactly why, but how.
Coil zippers aren't like that. They are made with less toothiness, and work more magically, which might be one reason that they fail for no reason that any sane person can comprehend.
True, they tend to be more delicate, and are used on lightweight clothing more than heavy stuff like pack pockets or jeans flies, but some are more robust than that too.
Everything does wear out with use, coil zippers too, and in the case of a coil zipper, by the time the shiny newness is gone the zipper gets balky, snags, bunches, and eventually comes apart.
There must be a minuscule timer mechanism built in somewhere (it seems like it might work this way with all zippers) so that any catastrophe happens exactly when that zipper is needed most.
It's enough to make anyone, let alone a hiker, wind up into a coil and hiss for a while.
But that attracts snakes, so it's not a good idea either. Looks like you're screwed then.
Source: How to talk in the woods.
We few, we grumpy few, we rumply-hat geezers say to you Effort or Eff it. No sniveling.