It's sexually suggestive and inviting as previously stated. Whether that be because its movments and body parts involved promote it, or the society we live in will percieve it as such and use it for nefarious purposes.
For young girls who do this for attention it promotes hyper sexuality among peers and onlookers. The problem is that at such tender ages many females are not ready to handle the consqunces of these actions. Sex is a critical part of life and everyone should experience it but only when they are ready to handle its implications. Twerking for young girls is hitting fast forward on this process.