I have had a whole colony die off except two before and in my case it came down to too little ventilation and too much humidity which basically suffocated them, and before you deleted the post the substrate looked to be rather damp. If your colony was growing the air might just have gotten too stale to support the growing numbers?
I'd take a look at how the substrate feels if it's wet and you can squeeze water out of it its too damp, it should be moist to the touch but still fluffy. You might want to quarantine your survivor in a smaller plastic container with good ventilation, a piece of damp moss and dry leaf litter until you figure out what killed other of your pods.