..and how do you prevent this?

I noticed my hair would try to do this when longer, and I hated how it looked. What I mean is when it would separate into "chunks" that would try to do their own thing and just look like a scraggly mess in general. It would tend to happen below the neck. How does one get the hair not to do this, and why does some people's hair do it while others seem to have nice hair that stays put all day long in one flowing section?