Hi Abdul! This is still an issue in the literature. Some patients may have dystonia of the abdominal muscles but this is a minor mechanism, according to new data. Myopathy of extensor muscles is a major mechanism, but stil we don't know what causes this myopathy. Recently, it was pointed out how patients with camptocormia have more frequently vertebral diseases or ostheoporosis, but we don't know yet if this is the consequence of abnormal postures or a precipitanting factor. 

A new review on the topis is this:

Srivanitchapoom P, Hallett M

Camptocormia in Parkinson's disease: definition, epidemiology, pathogenesis and treatment modalities.

J Neurol Neurosurg Psychiatry. 2016 Jan;87(1):75-85. doi: 10.1136/jnnp-2014-310049. Epub 2015 Apr 20.

For my experience , i think it's myopathy of paraspinals.

Best regard

Michel

It all begins with a postural forward head that drags the thorax down. Work for a  military posture early on.