How relaxed a person is around a snake---particularly when handling it---plays a huge part in how the snake will behave (or misbehave, lol). I believe a lot of it has to do with smell. And remember that snakes also detect heat, which I'm sure plays a huge role as well. If someone is nervous and attempts to handle a snake, the snake will be able to interpret all sorts of stimuli which us humans may not even think about. Things like perspiration, increased blood pressure, breathing pattern/rhythm. When you think about it, all of these things (and likely many more!) come into play and, I believe (though I'm by no means an expert) play a role in at least some measure. And of course, how physically 'tense' a person is while attempting to handle a snake will surely be felt by the snake as well---just as us humans are able to detect how tensed-up or relaxed the snake is just by touch.
Awful lot of things to consider!
And by the way, I share this based on working with hundreds of snakes on a daily basis and handling dozens daily at the breeding facility I work at.
Vine