Can my Grandpa get custody of me without reason?
Not knowing if your Grandfather fought for custody or if your parents have just asked him to take over custody would be the question here. If he fought and won custody...there would be a reason. If your parents decided to turn custody over to him...there also would be a reason. Now is the time to sit with your parents without drama ( I know this would be hard) but if want your answer you need to sit quietly and as them why.
If your parents choose to relinquish custody, and you are a minor, I believe they can transfer the custody to another guardian, such as your grandfather.