From what i've gathered that they've said about it, it has to do with the game taking data from their servers. Since your Destiny account is in their servers, not your system, it has to constantly drag info from the servers to your system when switching instances. Them stopping you from checking your inventory has to do with ensuring the integrity of your account in the servers, as people have lost items before by switching items in a certain window of what it now doesn't allow you to. If you switched an item during that time, there was the very real possibility of the item being lost due to certain discrepancies between what you have and what the server has. For example, you switch gun x(equipped) with gun y( not equipped), resulting in y being equipped in the process. It may receive that y was equipped, but x was never accounted for, so poof, no more gun x. Understand im not condoning that we should have these interruptions, im only putting out why it exists, as has been explained by bungie. Where this is from, i can't source as i dont know the source, so believe it, don't, up to you.The point im making is, that if what i've put down is of semblence of what goes on behind the code while throwing around your accounts data in the server from server to system and back, then they would have to create a more streamlined, singular block of you not being in your menu, possibly with a prompt while flying as a circular node with a red X for "you cant go in yet", a green check for "you are good now", as well as a warning ticking down of "prepare to be kicked from your menu", similar in position and function to going to orbit. TL:DR Bungie has those kicks for technical reasons, else you might lose your gear if switching gear when those kick prompts would have been active. It is risky business messing with that, because coding is... unique; stress that idea, coding is the most fickle of fickle mistresses. Possible solution is a singular block of time not being in your inventory, and background processes working here, with visuals in corners while things are going on that say you are about to be kicked from your menus, no inventory access, or inventory accessible, basically.

Why are you in your menu in a crucible game

You guys do realize that this is probably impossible, right? I'm mean come on it [i]has[/i] to be technical issue. You don't actually believe Bungie intentionally implemented a mechanic that forces players out of their menus, do you? Think about when this happens, while you're in transit between locations. Remember when they first tried adding the app feature that let you move stuff between your character and your vault, how lots of people were losing items? Bungie isolated the main cause of this as being people moving gear while in transit between locations. The answer seems pretty clear to me, the system just isn't able to handle you moving stuff around while it's moving you around.

I think this is probably the only remaining grievance I have with destiny, to think I don't have something bigger to complain about is most probably a good thing, but yeah, I find this so annoying, I'm sick of being taken out of the menu