Why do my songs rearrange themselves automatically?

Occasionally, the songs on an album on my iPhone will rearrange themselves by some unknown means. Today I went to listen to an album and three songs had been moved all the way to the bottom of the album, putting the numbers completely out of order. This has happened to me several times already and it's really annoying. I have tried deleting the song and downloading it again from a family member's purchased songs. It just goes right back to the out-of-place spot. Thanks for any help