Why would a well-fed, watered St Bernard dog living with other St Bernards keep running away?

A family member keeps 3 St Bernards. One is old and arthritic; the other one is a year old puppy. The puppy’s sister has a broken leg that was bitten by a copperhead but runs away continually. She was found five miles from home today. They live on seventeen acres but wasn’t chained up today out of kindness; she is chained up about half the time. Why does she keep running off? She has her elderly mother there who probably is the one who broke her leg by biting it. She will stay at home now for a few days to a week then wander off again and be on someone’s porch or be seen romping with a pack of dogs. She was spayed.