There are two possibilities, I see, and only one is avoidable. The first is, when I experience this (and I do fairly frequently), it is because I am tired (not fresh to the job in the morning) and what I am reading is boring. My job requires a great deal of non-fiction reading that is not intended to be entertaining, and much non-story reading, like hundreds of pages of specifications or instructions for a large project.
If the problem is internal to you (you don't have the energy right now for imagination and understanding), automatic reading may be an internal mechanism we use to "get to something interesting or surprising".
For example, in a specification for some new electronic component there can be several pages of what is usually boiler-plate that is always the same on every part like it. But they must be given, and must be read, just to ensure they are NOT different. It is easy to find myself half way down a page without any recall of reading the first half, probably because there was nothing new in the first half, but I have to guard against "reading what I expected to see" instead of the actual text of what was written: So I start over.
We can't solve the reader's internal problems with reading fatigue. You can solve the reader's problem with boring text.
Most of this WritersStackExchange is devoted to that problem :-), so I can't fully explain how to fix it here.
In general, exposition is often boring. Dialogue that has no conflict in it is boring. Dialogue has conflict when it has the potential to change how characters think, feel or behave, going forward.
"Negative consequence" conflict is fine, that includes arguments, or resistance to accepting information, or resistance to persuasion, favors, or despaired begging.
Dialogue has conflict when there are stakes, a risk: Say the reader knows Allen is deceiving Brittney: The dialogue can be very routine and mundane, but the reader knows there is a risk Allen will be found out, and may be rooting for Allen to succeed, or rooting for Brittney to realize it, and not knowing how that will turn out is the hook to keep their attention.
"Positive consequence" conflict is fine, too. That includes just funny dialogue. Or when the reader knows Allen has something positive to tell or ask Brittney, and is looking forward to the reveal. The marriage proposal at the end of "When Harry Met Sally". Much of that movie is dialogue, but note how their phone conversations change either Harry or Sally. In their first conversations, idealistic Sally rejects Harry's cynical philosophy. In the famous diner scene where she fakes an orgasm, it is to refute a claim of Harry's. One of the most iconic bits of dialogue in Hollywood comes at the end of that scene, when a waiter asks another woman in the diner for her order. The woman is captivated by Sally's orgasmic performance and nods toward her, and says, "I'll have what's she's having." That is just funny. It doesn't make any difference in the story (okay it does but I won't go into it), but it makes the audience laugh and ends that scene.
Dialogue is boring without conflict. That is why using dialogue for Allen to explain something to Bob is boring, if Bob is just nodding and shrugging and saying, "Okay, I could see that," twenty times.
Long passages of exposition can be boring too. It was expected a century ago, but nowadays it is why most people put down Moby Dick within the hour.
Your writing should match the reading attention span of readers, which is typically a hundred to three hundred words. The average reader consumes 200 words per minute (a novel typically has 250 words on a page), people that read a lot (like regular novel readers, college students, people with jobs that require reading) average about 300 words per minute. Speed readers can have over 90% comprehension at five times that rate, 1500 words per minute. In comprehension tests, average untrained readers have about 50% comprehension of details provided in a 1000 word passage. All of those are diminished by fatigue.
All of that said: change gears often. If you exceed about a page (250 words) without conflict of some sort, you are at risk losing the reader's attention.
(By "conflict" I hope it is clear I mean making the reader wonder what happens next in the very near future, like within another page: Such texts are often called "page turners".)
For fatigue in yourself, teach yourself to take a twenty minute nap. I did! You can't do anything about physical fatigue in a reader, but you can be creative and find ways to pump your story full of conflicts, risk, surprises, betrayals, fights and struggles and hardships. Then, you may not be able to avoid exposition that has almost no conflict. Some little history lessons or explanations are just necessary. But try hard to keep it under 300 words, or even half that, before you find your way back to making the reader wonder how something will play out within the next page or two.