I'd hazard an educated guess at bad growing conditions - it looks as if this plant is in a pot, which presumably has drainage holes, sitting inside an outer container which probably does not. Succulents like to be kept on the dry side, preferring well drained conditions - if you've watered that too often, and not emptied the outer pot of all water twice after half an hour each time post watering, then its roots have been sitting in wet conditions. Most plants do not like that kind of environment, but succulents particularly loathe it.
It also seems that the outer pot is too deep, meaning the plant itself is sitting too low in it - this will block airflow around the plant. Moving air reduces dampness and the likelihood of fungal conditions, although I'm not suggesting you should keep all houseplants near a wide open window in cold conditions.
Watering houseplants is a bit of an art - in general, you water only when the surface of the growing medium is dry to the touch, but not so dry it's shrunken from the sides of the pot. When you water, do it well, let the plant sit for half an hour, then empty any outer pot. If its still sopping wet, you need to return and empty the outer pot or tray again, and maybe again, till there's no water present which is still draining.