Was your arm simply coming forward, or were you rotating your upper body to face forward? Probably the latter.
If Snowolf comes on-thread he'll fill you in with more detail, but basically if you've already rotated your body forward, you'll find that your board wants to turn that way to follow, making heelside turns not only easy but unintended -- like you go over a bump and lose contact with the snow, when you come down you'll be in falling leaf position, followed immediately by cab snow-angel. At the same time, toeside turns become more difficult and frankly more likely to cause a muscle pull IMO.
One trick some people use is to grab their pants legs with both hands so the upper body stays parallel to the board. But do something. You have to break this habit.
I know you think you thought you knew what you thought I said,
but I'm not sure you understood what you thought I meant.