i keep the snowboard in my board bag in my closet so do you think there is any other reason for rust forming on the edges?
The primary reason for rust is O2. Oxidation. Pretty much if your board is exposed to oxygen and there is humidity in the air, more in the south and coasts then up in the mountains, the edges will develop rust no matter what you do. Light sanding should get it off no problem. Keeping it indoors is probably the best you can do, but considering we live on earth, rust is a natural part of having exposed steel.