too much shit in diaper. he leans as a counterweight

He doesn't have the core body strength to compensate for the lifts he's wearing.

I've read that it can be a symptom of balance loss due to dementia.

He’s an amputeed centaur.

Not a doctor, but I favor some sort of fronto-temporal neurological issue.

According to Cytowic, 'It is true that individuals who have balance and gait issues similar to those observed in Trump can have degenerative brain disease in the frontal lobes, such as fronto-temporal dementia or Pick's Disease. Other possibilities are normal pressure hydrocephalus, sensory ataxia, peripheral neuropathy, small lacunar strokes in the basal ganglia, supranuclear palsy, the effects of too many medications, and Parkinson's Disease, which can begin on one side and also show early cognitive impairment.'

Didn't he always? It's been his signature posture for-like-ever.

Leaning forward hides your gut.