This is the big problem with crickets. Crickets close to 1", even around 3/4" don't live for too long regardless of care. So if you bought them at 1/2"+ size you probably got some older ones in your batch.
How many were in the tub to start with? Maybe it was TOO crowded. Once they die off like that, it isn't easy to make chain of death stop.
This is the main reason I stopped with crickets. I had to buy 4,000/5,000 a month and cleaning 2 times a day, I would end up losing 1/3 by the time I got to feed them. Was not worth the effort.