Are attachment problems a sign of child abuse?

Often not. Although child abuse certainly can lead to attachment problems, most children with attachment issues were not abused. Most commonly, it results from a mismatch of child needs and parenting style. Other contributing factors can be adoption or parent illness or death when children are very young.
Not Necessarily. Attachment problems can arise from a wide range of innate predispositions in the child interacting with relationship factors stemming from the environment. For example, a child who is genetically loaded mental illness, being raised by a mother struggling with severe depression during the child's early development, can potentially give rise to attachment difficulties.