Don't forget, regardless of ambient humidity you're burning moist leaves. That'll create steam in the cigar all by itself, which, as been pointed out, causes expansion of the filler leaves. And, of course, higher humidity just compounds that, whether we're talking ambient conditions or the RH of the cigar.
Also look to how tightly packed the filler is, leaving less room for expansion, as well as how delicate the wrapper may be. That right there should tell you why NC's may tend to be less susceptible to splitting than CC's (though certainly not immune). However, there also may be a mitigating factor as well, in that reduced ventilation through a more densely packed cigar could tend to produce less steam due to limiting the heat and speed of the burn.
I smoke in moderation. Only one cigar at a time. - Mark Twain