I'm not really sure what you're asking. Nobody goes in thinking - there's a 50% chance I'm out of here in the next two years.
There isn't a risk tolerance or some secondary thought process. Sometimes a kid gets to campus and isn't good enough. Sometimes there is a lingering injury where they never get completely right and it reflects in their performance. Sometimes the player and the coach bump heads. Sometimes new coach(es) are brought in and they need to clean house. Sometimes a player never belonged. Sometimes a kid doesn't make the travel roster and says if I went somewhere else I'd be traveling. Sometimes a kid can't afford the school anymore. Sometimes a scholarship isn't renewed at the same rate.
I've seen a lot transfer because they pulled a muscle or strained something in the fall and got shut down. By the time they were healthy, others had passed them by or they didn't show enough in the short time they were back.
It's not something to worry about until it is