Usually collapsing is because players are pushing off of the rear leg...so the spine and rotation are centered on the back leg but we look at it on video relative to the front leg, so it looks collapsed.
Basically, pushing into rotation from the back leg is the most common cause of collapsing lead shoulder IMO. If you get onto the front leg before throwing, especially with the spine over the front leg and right shoulder closer to the target than your front hip, then your right arm should be pulled correctly rather than collapsing inward. Think like shoulder pulling the arm along for the ride vs. chest getting pushed in toward an extended arm.