When a shulker (the first shulker spawned/loaded in) opens, so too do all shulker boxes in the entire world, regardless of what dimension you're in. If that shulker gets killed, all the shulker boxes sync up with the secondmost shulker loaded in, and so on and so forth, and if there are no more shulkers, they stay in whatever position the last shulker was in when it was opened. Furthermore, if a shulker is killed while fully open, and then you open a shulker box that is fully opened, the shulker box grows in size, and the animation starts from where it is currently. (It's hard to tell but that is depicted in the screenshot named "2020-11-04_14.06.28")