These plants are rainforest jungle epiphytes, not dry desert dwellers, the handle 'cactus' can be misleading in that regard. It's extremely stressful for them to dry out. I'd be more suspicious of that being the cause.
However, if the soil they are in doesn't have ample air in it, that could lead to rotting if kept as moist as they prefer. After moving all of my plants away from peaty potting soil, I only need to let the true cacti become dry to avoid rotting. It's much easier when your soil isn't your main enemy, and when the one thing plants need for us to do for them (add water,) isn't also the thing that kills them.
