This is actually a very good question. I've always placed different maps into groups but haven't ever really considered why they belong where they belong. For example, I considered maps like Guardian, the Pit, and Lockout highly competitive maps... while I considered maps like High Ground, Zanzibar, and Headlong more casual maps.
I guess perhaps I have always considered the arena like styles and pure balance of maps like lockout and pit to be a more highly competitive atmosphere than the casual competitive ones. Maps like Lockout feel as though they are designed specifically to hold battle upon. All of the structures are seemingly built around LoS, Risk vs. Reward, and Weapon Placement. While, on the other hand, if you look at maps like High Ground it seems less like an arena and more like it wasn't meant to have battles fought upon it.