My philosophy on yoga technique is pretty simple. All poses are a reflection of tadasana, or mountain pose. A third name for standing up straight is samastitihi in Sanskrit, or equal standing. As a teacher I use the position of a students feet as a diagnostic tool for what the rest of the body is doing.
Standing in tadasana we hope to have balance in the weight of the feet—not too much to the inside, outside, heel or forefoot—even though I am always teaching students to accentuate the grounding of the foot through the mound of the big toe.
The essence of what I teach is everyone leans backwards and tucks their pelvis under and everyone needs to stop. This happens in Tadasana, triangle pose, waiting for the bus and walking down the street. It is a sad and sorry thing. And while the yoga room is the place this exact habit should be broken, it is instead exacerbated by endless well meaning teachers delivering poorly articulated instructions.
Even thought the relationship of the shin and the foot of both legs in trikonasana are very different than in tadasana, I don’t think the feet change all that much. There is more weight in the heel of the front foot and the forefoot of the back foot but the inner and outer foot should remain relatively the same.
I always set my legs first in triangle pose, aligning the feet and knees and deepening the groins trying to get both legs in line with the hips. To do this I will often let my upper body lean forward a little letting my legs set back where I want them. Once my feet and legs are set I can then make an attempt to align my trunk and I can use the feet to tell if I am going too far.
Letting the upper body lean forward takes the pelvis out of its habitually tucked position. Pay attention to balancing the weight of the inner and outer foot as you move the upper body deeper into the pose. We tend to lean backwards when we stand because of habitually tight lower back muscles. To go deeper into triangle pose the trunk should move into position evenly, instead many students go deeper by shortening the back body and splaying the front body open.
This is probably what you have done in triangle pose if the weight in your feet shifts to the outer foot. Bring weight back to the whole foot, even if you can’t get that deep in the pose, and try to advance with a balanced trunk keeping the weight even in the feet.