Why? This only became apparent to me in the hub of human multitasking, the New York City Subway. People do a lot of reading on the subway, but in New York the majority uses iPads and other eReaders. When Steffi and I were riding the subway, I noticed a men who exhibited some odd behavior while perusing his iPad. His thumb seemed to have a strange twitch, almost a tremor, and hovered over a corner of his iPad going back and forth. Upon closer inspection it became clear to me that the man was not suffering from a strange neurological disorder, but was caught up in a twist of multitasking conflict. He was reading a book on his iPad, and his thumb was above the corner that turns the page.
Now, think about how you read books (at least it is how I read books). If your gaze approaches the end of the page, say the last few lines before the end, you already lift up the corner of the page in order to be able to turn it right at the moment you read the last word. If you wait with picking up the page until you have read the last word you will lose time and might even disturb the flow of whatever you are reading. So, what you are in fact doing is engage in multitasking. Picking up the page before reading the end of the page is not part of the standard reading process. It is not as if you always pick up the page, say, exactly three lines before the end. Instead there is a parallel monitoring process that itches to pick up the page as you get closer to the end. You don't do it with a newspaper, and when you read a text online you do something different (scrolling).
But now enter the iBook reader on the iPad. It looks very much like a real book, including a stack of pages below the current page you are reading. If you touch the corner of a page, it will go to the next. It is almost like a book, but not exactly. If you act exactly as you would act with a real book, you turn the page prematurely (happens to me all the time).
Now, our poor multitasker was probably halfway learning to use the iBook reader, and the tremor in his hand was produced by the normal book-reading process willing him to touch the corner, and the new iBook reader override trying to convince him not to. Because he was so engaged in the reading but maybe also monitoring the subway ride, he probably didn't even notice it himself.
This small example demonstrates that many of our behaviors are composed of several smaller task that we execute in parallel. When we read a text we have a text-processing process, but also a process that makes sure our eyes are fed words at the right pace. And even though text-processing is usually the same, the word-feeding process may differ depending on the circumstances.