I'd say it's a result of being cold, no moisture or fresh water or food. They also need to be kept warm and the tub needs to be humid. The best way to keep them long term is to line the bottom of the tub with a substrate such as vermiculite between 30cm and 50cm deep, provide eggs cartons for cover, a small plastic tub with dried dog food (the smaller type of dried food is the best and it can be topped up as required), some carrot or sweet potato (replaced as needed) and another small plastic tub with a piece of moist sponge (similar to that used for washing up but soap free) cut to fit snugly in the tub. Cut out a piece of the lid (about a 3rd the size of the lid) covering the tub containing the crickets and secure fly wire over the opening. Then set up a heat lamp with a 40w globe positioned over the top of the cut out section (this can be achieved by using a desk style lamp). The vermiculite and sponge should be lightly misted daily to maintain a humid and slightly moist environment. Just remember to remove the dog food so it doesn't get wet and NOT to soak the vermiculite just mist it enough to make it slightly damp.