Improper throwing mechanics is the one of the biggest, if not the biggest, issue I see with girls at all ages but in particular at 10U and 12U. Many of them look like they are throw or two away from blowing out their arms with the way they throw.
In my observations, growing bodies are also a contributor. Bones getting longer, and ligaments being forced to stretch while under the heavy load of a throwing athlete makes our girls even more susceptible to these issues.