Is hardtack supposed to break on it's own?

I cooked them at 300ºF for three hours and then took them out to cool. I then put them back in the oven at 250ºF for another three hours. Each time when they were cooling, I kept hearing something I can only describe as glass breaking.

I got alarmed thinking I left something on the stove or something had fallen off the counter and cracked. But it was just the hardtack. It may have been the uneven surface it was laying on as they cooled. Is that normal?