On the other end of the connection, there are physical servers and routers. Every once in a while they change how things are connected/deployed for maintenance, upgrades, etc.

Pretty much, I have my cable modem on continuous power and it will keep the same address pretty much forever. Two times it changed is when I had a 48 hour power outage and shut everything down, and the other time was maintenance at the cable companies side where they rebooted their equipment.