Sleepless night

Why do smoke alarm batteries only give out in the middle of the night and start “chirping”?
The dogs were not pleased with this intrusion of their sleep as were we. Scrambling to find a new battery and a step stool to reach it, Ron replaced the battery. In the meantime Bali was in full panic attack mode. It took awhile to calm her down but she wouldn’t go back to sleep in her bed.
She and Tahoe slept with us for the remainder of the night. Not much sleep was had by us however.