User FalseDmitriy on Reddit gave a good explanation....
So I'm pretty sure I know exactly what happened here. I haven't seen anyone else post about this, but as a teacher who works with struggling readers, I know that highly literate people (including most general-level teachers) have a hard time understanding how someone like this approaches written text, since for many of us reading comes so naturally. From my perspective it's pretty easy to see why Trump said this weird thing, given what we know about him. We know:
Donald Trump does not read well. Like most of the students I work with, he avoids reading both because he wants to avoid being embarrassed, and because reading costs him a lot more mental energy than for proficient readers. We know from lots of different reports that his staff does not give him anything long or complex to read, because of this avoidance.
For this reason, when Trump does have to read something out loud, it is clear that he is not processing the meaning of what he is saying. For a struggling reader, all their concentration goes into pronouncing the words out loud, and simultaneously processing the meaning is very difficult. We see this when is giving a prepared speech and mispronounces a word in a way that makes no sense. A proficient reader would immediately stop and self-correct. Trump often doesn't, because he is not processing what he is saying. Other times I know I've heard him notice his mistake, but instead of correcting it, he covers it up with a bit of lame word-play, pretending that the mistake was intentional. I can't think of any specific examples of this, but I know I've heard him do it. (Edit) snatchi found some examples: "through their lives... and though their lives." "authority... and authoritarian powers." "They sacrifice every day for the furniture... and future of our children." It's Trump's go-to move when he misreads a word.
There are other times when he reacts to a line in his speech like he hasn't heard it before. He noticeably stops and inserts a comment of his own before going back to the reading. He does not know how to gracefully glide between reading and impromptu speaking, since reading is so unnatural for him.
Trump also has a relatively small vocabulary. Remember his remarks about "the oranges of the Mueller report." He was parroting something that he had heard before, but not having a firm grasp of the word "origins," he used a more familiar word instead, because that was how his mind remembered the word.
The speech he was giving made heavy use of language from "The Star Spangled Banner." For many struggling readers, this would be helpful, since it would rely on familiar chunks of language that would reduce the mental load of reading it. However, we've seen that Trump does not know the words to the anthem. He has tried and failed to sing along with it but couldn't fake it very well.
Keeping all that in mind, let's look at what he said:
Our army manned the air, it rammed the ramparts, it took over the airports, it did everything it had to do, and at Fort McHenry, under the rockets’ red glare, it had nothing but victory.
Based on my experience, here's what I think happened, step by step.
Our army manned the air
Here I think it's likely that Trump skipped a line on his teleprompter. The line was probably "manned the ramparts," and later on I'm guessing there was a reference to "bombs bursting in air." We all do this sometimes, but struggling readers do it a whole lot more. And furthermore, when a proficient reader makes this mistake they can quickly self-correct, but someone like Trump, who is not totally processing the meaning of what he is reading, can get totally derailed when they do this.
it rammed the ramparts
Trump seems to have noticed that "manned the air" was a mistake, and he went back to do the line over. But he got "manned" and "ramparts" mixed up, so it came out as "rammed." But he's immediately fallen into another pit: the word "ramparts." He doesn't know what it means. It's a very uncommon word that most Americans only know from this line in "The Star Spangled Banner." Trump, however, doesn't even know that, since he has never learned the words to the song. So I think that at this point, already a little flustered from covering up his last mistake, he thinks he has mis-read another word. "Ramparts?" I must have misread something, he thinks to himself.
it took over the airports
This is a repair strategy that Trump has used in the past. Mess up a word? Pretend it was the first in a sequence of rhyming or similar words and carry on from there. What's a word he knows that sounds like ramparts? Airports. And "air" was already on his mind from just before, when he accidentally read "manned the air." So they manned the ramparts, they took over the airports. He's hoping that nobody will notice. It's worked before.
it did everything it had to do
This sounds like an impromptu comment that he inserted into the written text. It uses the simple and non-specific language that he is known for in his impromptu speeches. The comment bought him a second where he could find his place after getting completely lost before.
and at Fort McHenry, under the rockets’ red glare, it had nothing but victory.
And now he's found his place again. He's back to the written speech that uses lines from "The Star Spangled Banner." He might not even realize how ridiculous his last few sentences have sounded, since again, he's not really able to process the meaning of what he is saying.
My kiddos who are in this situation have a hard time. I and their other teachers have to work really hard to help them learn strategies to overcome these difficulties with the way they process written text. It requires just as much hard work on the kids' part. I strongly suspect that Donald Trump never went through this process and remains in a not fully literate state. Usually we're afraid that someone who graduates with this level of reading ability will have very limited career prospects in the future.