Concur with speedjason and other's too If your body is facing forward instead of aligned with your board,your back leg wants to pull out hence your tail wants to slide out. Keep your front/back knee bend equally and stay centered. The only time your tail needs to slide out on the bottom of a run is when your slowing down and stopping.
2015 Rossi One Mag 159
2015 Capita Charlie Slasher 161
It's a rossignol Taipan 2011. Bindings 15 -15, highbacks 3.
I just saw a post about adjusting the highbacks to be parallel with the board edge so I just adjusted that now.
But rotating the highbacks will not "make" your tail not slip out. It's all in your body allignment and edge pressure.
What you have to do is bend your knees and apply pressure to one of the edges so that the board is 'tracking' instead of just 'floating'. Keep your shoulders and hip inline with the board (and do not try to bring your hip to make yourself face the slope), and have more weight on your front foot to keep your body perpendicular to the slope and initiate turns. Direct the turns starting from your front foot & knees, then upper body..... and then in the other direction.
(Some people teach starting from head/upper body then knees/feet, others teach starting from feet... choose your flavour)
Yeh looks like your opening up and weight shifted back. Lots of good tips already given. I'll just add as you get over your fear it'll get easier keeping aligned. Most of us who haven't taken lessons or gotten instructions have been there. Take what you learn here and apply it when you're out.