IMO a lot of kids suffer a butt out condition due to an issue with their drive mechanic. Kids who have good glute usage will go from the "k" (bent rear hip) to reverse "k" (extended rear hip) position naturally. Kids who do not have good glute usage will drive from the rubber using primarily quads and calves (late push pattern). These kids will tend to end up in a butt out condition due to their rear hip not fully extending during drive from rubber.
Secondary effects of this are lack of mid section propulsion not yielding good weight transfer into plant, lack of propulsion forces pitcher to reach out with front leg, late push pattern from leg/calf after they start to close will cause the rear end to kick-out even more.
Late rear hip extension is a symptom which seems to be a result of the dragging rear leg. It is easy for this issue to be masked by a good front foot plant and cracking the nut which will result in a tall posture at release even though they were butt out during the important part of the drive phase.
Check out how early Tatum Edwards' rear hip extends (great clip knightsb!). She is driving from the middle out.
Good rear hip extension vs. Bad rear hip extension (resulting in butt out)
How are you training god hip extension.
A way that I have found to get them to do this without focusing on the hip is to tell the to FEEL like they are driving straight out from launch out