There is no reason why both explanations cannot be true. It is not uncommon to do something for more than one reason.
Exodus 22:11 tells us the Israelites should rest because God rested on the seventh day (reason for the commandment )
Deuteronomy 5:15 tells the Israelites that God commanded them to keep the Sabbath because of their deliverance from Egypt (tells us a good reason why they should keep it)