Unstable midfeet usually cause ankles to waffle around, and also strange things to happen in the forefoot. Not always spreading out, but definitely issues. I have an exceptionally high arch, and loose foot structure, but skiing without my custom footbeds does very little to the overall fit of a boot.
9 times out of 10, shinbang is caused by TWO things. Improper form, and boots that are too big. Although not always connected to park skiing, it is an easy way to encounter that problem.
If a boot is sized correctly, and you are landing backseat/skiing with poor form, you will get very sore knees, and sometimes a bruise on your shin, but it is typically different than "shin bang" which is almost always related to boot size.
P.S. I wouldn't call me a bootfitter, more like an intern, lol.