I just fed my 2 corn snakes five live hoppers each about 5 hours ago. I like to leave them in their feeding boxes for a while to make sure they eat everything. I went back to put them back in their cage and one of them was dead. No outer signs of injury or stress. Now I know it's quite ridiculous to ask for an exact cause of death on here, but does anyone have any ideas how this happened or experience with this?
You won't necessarily see outward signs of stress. Which I believe is actually the case here. Plain and simple, and I can't stress this enough, you should never feed your snake live food. If for some reason you absolutely have to, (i.e. you've tried every other option), you should only do so under strict supervision. You should still supervise feedings with frozen foods anyways.
Here is a picture of a ball python after a rat was put in it's cage overnight. Warning! It is quite gruesome. Original post with this image is found here.
Now, your snake might not show bite marks, but that doesn't mean that the mice didn't fight back. I mean, why wouldn't they? They're fighting for their lives. Eating live prey can be one of the most stressful things a snake does, and that's why they live so long in captivity, because they don't have to eat live prey. Not to mention, having multiple mice in a feeding container, it's not like the other mice are going to stand back while the snake is eating one of them. It's pretty hardwired into rodents to attack snakes if they see them.
Some states have made it illegal to feed snakes live prey. Partly because it can be seen as cruel to the feeder animals, but mostly because of how painful it is for the snake to be torn apart by the rodent.
Snakes are ambush predators. They sneak up on their prey and catch them unawares. It's practically impossible to ensure that the snake has the upper hand in captivity. Whether you put the snake or mice in first, the mice will know the snake is there. If not by sight, then by smell. Basically, if you want to ensure the snake is able to capture it's food properly, you'd have to let the mice go in your house, and let the snake go after them. Then, you no longer have a pet.
What probably happened, is that the mouse was struggling as the snake was swallowing it, and it's claws tore into the snake's insides.
The other thing I'm worried about, is how many mice your feeding your snake. It sounds like you put 2 snakes and 10 mice into the same container and left them to fend for themselves. Corn snakes are opportunistic feeders, and will eat as much as they can, in case there's a shortage of food later on. There wouldn't be, since they're pets, but they don't understand that - It's instinctual. Unless they're in separate containers, it's quite probable that they get different amounts of food each feeding time.
You don't say the age or size of the snakes. But even for a grown adult corn snake, they shouldn't eat multiple mice at one time (definitely not 5 at a time). Even though the mice are small, you need to adjust their feeding schedule so you're feeding them more often, not all at once. Even feeding multiple frozen mice to a snake can cause internal damage.