At the following link

there is a list of the results of childhood neglect and abuse.

"Poor muscle tone, poor motor control, delays in gross and fine motor development and coordination, failure to develop and perfect basic motor skills and lack of physical coordination" can result from neglect, from malnutrition or from repeatedly being hit in the head or shaken as an infant or minor child.

I think the studies about adults who are physically clumsy should include interviews or investigations into the individual's background; I'm willing to bet that chronically clumsy adults have some form of organic brain damage or inner-ear injury, and that in a certain percentage of cases this was caused by parental/primary caregiver abuse or neglect throughout the individual's formative years.