The elderly usually sleep more often due to things such as SDB, circadian rhythm disturbances, RLS/PLMS, RBD, depression and other psychiatric disorders, medical illness, and medications that can cause the elderly to feel more tired in the day.
When they are more tired in the day the elderly then sleep more to compensate because of the tiredness.
Also sometimes as a person gets older they suffer from joint pain, back pain and even insomnia and that makes it harder for those elderly people to sleep.
Then since the elderly person sleeps less during the night they need to compensate and get more sleep through a nap during the day.
Sometimes an elderly person may also just be bored and fall asleep because they don't have much more else to do.