I can explain you with examples:
"Yo caigo" or "Me caigo" both are action expressions that mean you are falling, more popular is to say: "Me estoy cayendo". For example when you are on a slippery floor, you lose your balance and fall to the ground. Although you do not use this expression very much, because when you see yourself, you are already on the floor and what you can say is: "Me caí" or "I fell". While "Se me cae" is more used to say that it is an object that falls out of your hands, because it can be heavy. For example, when you move a piece of furniture or a heavy box from one place to another, you can say, "se me cae" to the another person, as a way of asking for help.