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As previously said, stable is a relative term. The portal is less of an doorway and more a mass of, to quote a certain doctor, wibbly wobbly timey wimey stuff. Think of a fluttering curtain. If you hit that curtain at the wrong angle, your ship suffers in some horrible or more horrible way. While the flutterings of the curtain are seemingly random they can be calculated to some degree of accuracy using very complex mathematics. (I imagine multivariable calculus combined with trigonometry). If correct, you get a flight vector and, given a passable pilot check, pass harmlessly through the curtain. Voilá, portal jump! If incorrect or shoddily flown, bad stuff happens. But who knows, you could just fly in and pray to the icons that you hit it in just the right way as to not die.

This analogy works well with explaining unstable portals too. "Stable portals" are comparable to the curtain in a regular window on a non-descript summer day. Sometimes there a little gust of wind but it mostly just tapers back and forth lazily as the wind moves oútside the house. An unstable portal is a curtain at a balcony door during a storm when you also have the kitchen window wide open, there's horrible crossdraft and sometimes creatures and rain come crashing through, that curtain is fluttering like crazy now and hitting it "just right" is that much harder.