It’s pretty hard to explain exactly “what” is causing the massive instability of the connection the realm worlds on Switch have, but a few instances that are recurring are opening the inventory or a table / any UI will cause the whole game to suddenly crash, and I’ve also just broken blocks or gone through nether portals and the whole game crashes. I have a friend who also plays on the realm who also uses Switch Edition and it also spontaneously crashes in a similar fashion, it doesn’t seem to crash with Windows 10 edition players though.