I tried to use my decorating triangle for the first time yesterday. It's like the buttercream snagged on the triangle and I ended up with really choppy lines. Did I use it wrong? I had let the BC crust and I smoothed before I used it.

I find it works best if I use the triangle before the icing has crusted. HTH

You might be pressing too hard into the cake. You really just skim the surface of the icing.
Is it a plastic one? Or metal?
How thick is the icing on the cake? You probably need to make it a bit thicker.