The medical journal Stat is comparing the President's speech pattern from years ago with now and wondering if the President is sliding into dementia.
There is no doubt that Trump used to speak thoughtfully and in complete sentences as a younger man. He appeared frequently on radio and TV, and appeared to speak well. See here: 📹 https://goo.gl/IjNEYU
Of late, he appears not to complete his sentences, jumps from thought to thought (mid-sentence even). As the article says:
Now, Trump’s vocabulary is simpler. He repeats himself over and over, and lurches from one subject to an unrelated one, as in this answer during an interview with the Associated Press last month:
“People want the border wall. My base definitely wants the border wall, my base really wants it — you’ve been to many of the rallies. OK, the thing they want more than anything is the wall. My base, which is a big base; I think my base is 45 percent. You know, it’s funny. The Democrats, they have a big advantage in the Electoral College. Big, big, big advantage. … The Electoral College is very difficult for a Republican to win, and I will tell you, the people want to see it. They want to see the wall.”
He might be simplifying his language and delivery as a strategic matter. His campaign speeches have been simple to follow and understand. Perhaps he thinks people will think him elitist if he spoke the way he used to.
It could also be "cognitive decline." As Trump ages, his brain functions are expected to decline, and some doctors think he is suffering from cognitive decline. A loss of vocabulary is considered a symptom if cognitive decline.
We would never know of course, since Trump has never released the results of a complete medical checkup, something all other Presidents do as a matter of course.