I think my favorite sport in the Olympics is the one in which you make your way through the snow, you stop, you shoot a gun, and then you continue on. In most of the world, it is known as the biathlon, except in New York City, where it is known as winter. ~Michael Ventre, L.A. Daily News
I think it looks like you're not popping, then turning 90, you're rushing it and sort of doing both simultaneously, also it looks like you're looking at the pole and thus sliding off to that side. try and pop, then spin 90, focus on the end of the rail, and get a bit of grip with the uphill edge of your right ski as if you were prepping for front 2 out—just not as aggressively–it'll prevent you from sliding off to the left. try and slide the rail with the balls of your feet rather than dead center or more on your heels.
you look fairly comfortable, and pretty balanced, I think with some very minor tweaks, and just some more practice, you'll be locked in. If I were you, I'd build an "ohsonotcool" lip/full on booter in this case on the outside of the stairs to the right so that you can slide the other railing your natural way without having to get on lip. Then, what's your money over there, knock it down and go back to the other railing. That pole, when you're better, will give you the opp to do some sick shit ala taps, and uberduper fast spin outs.