It's age-related incapacity. Like his father, he is becoming demented. Old Man Fred lasted at least eight years with Alzheimer's, and ultimately the brain capacity of a woodlouse.
Wiki offer
"In October 1991, Trump was diagnosed with "mild senile dementia", with his physician citing symptoms of "obvious memory decline in recent years" and "significant memory impairment". A few months later, another physician reported that Trump "did not know his birth date [or] age", amongst other difficulties. Mary L. Trump recounted that as her grandfather's dementia progressed, he failed to recognize people he had known for decades, including her and Donald. According to Fred III, his grandfather needed to be reminded why he was at Donald's 1993 wedding (to Maria Marples) despite being designated the best man. Donald claimed that he first noticed his father exhibit symptoms of Alzheimer's in the mid-1990s." He died in 1999.
I'd suggest that Donald Trump is mentally about where his father was in 1993, at the very best, and we just have to hope he gets fatal pneumonia sooner than his father did.