The "cognitive test" that Trump performed on is not at all in depth, and put simply couldn't tell us if Trump was in beginning stages of mental decline.
Here, take a similar test yourself. If you don't get a perfect score yourself, for the love of god see a doctor now.
Why the square quotes around "cognitive test"? The test is designed to detect cognitive deficits due to decline associated with aging and dementia.
Also,
"and put simply couldn't tell us if Trump was in beginning stages of mental decline."
Well, it is in fact designed to detect mental decline. I don't understand the need to deny he checks out as far as the Alzheimer's speculation is concerned. For now.
Really, I think the most obvious explanation is that the guy is clearly out of his depth in the office of the president, he's almost certainly involved in sketchy shit with regards to money laundering, and the prospect of it all coming out is starting to take a toll. Even if that wasn't true, the burden of the office itself is taking a toll. I guess I'm biased, but he clearly cannot cope. Given his mercurial and narcissistic tendencies, well, he's behaving just about like what I would expect.
His entire career can be characterized as not doing really well in really sketchy businesses, like real-estate heavily involving hotels and casinos. After that, he moved to a bunch of mostly failed ventures that merely used his name and were built on exaggeration and bombastic branding, even delving into the downright absurd, e.g. Trump Steak, Trump University. You have to give him credit for something, his reality-tv persona, but the guy has clearly been about bluster and bullshit his entire life, and isn't at all that smart.