When the US president grabbed Theresa May's hand, insiders blamed a combined fear of stairs and dirty handrails.
It's a theory that plays nicely into the hands of Downing Street: Theresa May provided stability and support for the US president in his time of need.
But is a phobia of stairs or ramps even a thing? And how does someone develop it?
This specific fear is commonly called "bathmophobia".
How do you diagnose a fear of stairs or slopes?
The word isn't in the DSM-5 (the Diagnostic and Statistical Manual of Mental Disorders, volume five) which is the main publication used to diagnose and categorise psychiatric disorders.
It's published by the American Psychiatric Association (APA).
But names for very specific phobias are often avoided by experts, says Dr Abigael San, a chartered clinical psychologist in north London.
She told Newsbeat that she'd never heard of it, but added: "People do come up with all sorts of weird and wonderful names for their specific phobias, and often we haven't heard of them."
Dr San thinks a fear of slopes or stairs would come under a broader fear of heights, and could be linked to fear of escalators for example - something that is apparently quite common.
Does Donald Trump really have it?
This particular rumour was first written about in newspapers like The Telegraph, who quoted government sources in Washington DC.
Politico quoted a British government source, while Theresa May's spokeswoman - quoted in The Times - appeared to back up the theory that it was the ramp, and not a sudden rush of emotion, that prompted the hand holding.
"They were walking along. There was an unseen ramp. He put out his hand. She took it."
There's no way of proving the fear either way, but Will Pavia in The Times noted a 2014 tweet from Donald Trump that seems to relate to the theory.
"The way President Obama runs down the stairs of Air Force 1, hopping & bobbing all the way, is so inelegant and unpresidential," he wrote on Twitter in 2014. "Do not fall."
Why do you get bathmophobia?
Like any phobia, whether someone develops it depends on biological and environmental factors.
Biological fears can be traced back hundreds or thousands of years. When we were living in the wild, for example, there was probably a very good reason to be scared of spiders, as some species are poisonous.
If, as Dr San believes, fear of stairs or slopes is linked to a fear of heights, then this has its origins in a somewhat rational fear - even if it's manifested in relation to something that isn't actually particularly dangerous.
Environmental fears on the other hand, are developed according to direct experiences you've had or witnessed in your life.
For example, someone with a fear of stairs might have seen an accident in that space, or witnessed someone they looked up to become very scared or wary in that situation.
What your fears say about you
The new US President has been open about his other phobia: a fear of germs.
In his 1997 book The Art of the Comeback, President Trump wrote about how much he hated shaking hands because of the risk of germs: "One of the curses of American society is the simple act of shaking hands, and the more successful and famous one becomes the worse this terrible custom seems to get.
"I happen to be a clean hands freak. I feel much better after I thoroughly wash my hands, which I do as much as possible."
Fear of germs is fairly common, generally among OCD sufferers, says Dr San, and she says of President Trump: "Both of those things [phobias] indicate something about an anxious presentation, which is really surprising."
However anxiety-related problems don't always present themselves in the way we might think.
"Anxiety is a raised arousal - if you can use that to fuel something, rather than run away, you can confront it and use the energy that it gives you," says Dr San.
For example, she has treated a very successful pop star who had severe stage fright, but used the anxiety to spur him on.
And this could be exactly what President Trump is able to do so well.