To paraphrase Uncle Bobby B, "The stan don't pick the bias; the bias pick the stan."
I don't think I've actively tried to "choose" a bias, when it's happened, it's just been a realisation, something that draws my (almost) undivided attention. There's no one trait I can pinpoint that does it; it's probably a coincidence that
most of my biases so far are main vocals (Jihyo, Woojin, Siyeon - Yuqi is an outlier there).
The only case I can pinpoint a moment that made me realise "it's you", was with Siyeon, when she did this:
Basically every time I see her or she opens her mouth just confirms it.