The expansion of the arms in triangle pose, or trikonasana, should come from the core of the body and unfortunately I see that it is often happening from the arms or even the hands. Triangle pose, like any other asana, has a lot going on. The grounding of the feet is key as well as the depth of the groins and the level of the spine to name a few. An equally important aspect of the shape is the alignment of the arms and the initiation of their opening.
I tend to do the pose with the arms stacked in a straight line from fingertip to fingertip so if you were to put a broom stick behind my back everything would be touching—the backs of the hands, the forearms, the upper arms and the shoulder blades. This is just my preference and I don’t think there is anything particularly wrong with taking the top arm into the back plane of the body. But it is of the utmost importance how you achieve that movement of the arm if that is what you are looking for.
Too much of life as well as yoga is happening from the extremities. In my walking program I try to show people how they tend to move from the legs or even the feet rather than from the core. I believe that a great deal of chronic pain that people suffer in the low back, hip groin and even the knees and neck come the this instigation of movement from the points furthest from the center rather than from the core itself.
It is easy to feel this in triangle. Set yourself up in the shape and move the top arm back from the arm itself and you will likely feel the greatest action at the junction of the arm and shoulder blade often feeling it as a stretch of the pectoral muscles. Then try to consciously engage your abdominals and move from the spine, starting all the way down at the tail bone. If this makes sense, the action will flow from the tailbone to the fingers tips without that feeling of stretch in the front of the shoulder joint.
So take the arm wherever you’d like but instigate the expansion of the body from the core rather than the extremities and hopefully it will feel like a more fully integrated pose.