It all boils down to something called dynamic form IDs. These are strings of data that Starfield uses to keep track of items in the game. As you move through the game's vast, lonely galaxy, you'll find yourself moving items around, taking gear off of deceased enemies, and leaving behind unwanted space junk. This creates more dynamic form IDs to keep track of what you've done with all your loot. The more there are, the more moving parts your save file has, and, as any coder will tell you, this means there are more ways it can crash.
Were the game working properly, these IDs would recycle themselves every so often, keeping your save file clean. However, for whatever reason, this refresh doesn't trigger in Starfield as is, increasing the likelihood of a crash