Since you were gone when they were being cared for, ANYTHING could have happened. But if you're suspecting it was the aspen, and it wasn't purchased at a pet store or reptile show, it could have been treated with pesticides or other harsh chemicals. or even been stored in the same warehouse as harsh chemicals and some spilled on that batch. 5 snakes don't just up and die like that. Even something like crypto wouldn't do that.