What was the cause of the legal conflicts that arose between Mexican and European-American landowners in the late 1880s?

The correct answer here would be the disputes over land ownership.

The rights of the Mexican landowners were guaranteed to them by the Treaty of Guadalupe Hidalgo which was signed and is taken for the end of the Mexican-American War. In the Mexican law the community owned most of the common lands but in the US individuals had the deed that they own the property. European settlers bought the public land and the Mexicans objected this which lead to trouble.