Hi again,
According to one insurance company's claims for dismastings, most occur in low windspeeds. This has to do with characteristics of metal, especially work-hardening, plus wave action, plus most boats are in relatively low winds most of the time. It is not because low windspeeds inherently make things break.
Likewise it is safe to assume that your backstay didn't break because you pulled on the tackle under way. It is far more likely that it had been approaching breaking for quite some time, and when you put the load on it (an unknown load, I will add), your effort exceeded the rope's capacity to stay together.
And I disagree; I think that it does "add up" that a series of factors resulted in an ostensibly stronger rope breaking, when wire in similar circumstances didn't. It was rope not optimal for standing rigging, it had an at best utilitarian splice; it was sized far too small for the load, it had been living, unprotected, in very harsh sunlight, and, perhaps most importantly, we don't even know what the rope was, so we can't make any judgement relative to its original quality. Too many variables.
For one chart on the expected behavior of Spectra in UV, see file:///C:/Users/Brion/Documents/Rigging%20Information/Rope/US%20Sailing%20Spectra%20UV%20chart.htm. You can find the original document, which is about lifelines, here:
http://www.ussailing.org/wp-content/...n%2020 14.pdf.
Fair leads,
Brion Toss