This is a big topic, and has occupied some of the best minds in the field for decades.
There are a number of theories as to the etiology of learning disabilities, and to some extent, all of them might be true for different people. As in some other kinds of brain disorders, learning disabilities tend to run in families. If one of your family members had a hard time reading, it is likely one or more of the children would have had the same problem. In my practice, it is not unusual for a parent to report the same problem! "Hey, I want to do those tests!" some of them say. And occasionally they will go through the same 7 to 8 hour battery, only to find out that they too have a learning disability—sometimes just like their child.
About 80 percent of those who are slow readers have a problem with phonological processing. This means that there is a problem with connecting the written word with the sound of that word. To many, this seems easy. But to the learning disabled individual, it is a struggle, especially with harder words.
There is also evidence that the brain in individuals with dyslexia functions differently than in those who are good readers. Two medical doctors at Yale University, Drs. Sally and Bennett Shaywitz, have been doing research and integrating some of the previous research in this area. Their work has illuminated the fact that individuals with dyslexia have a different brain organization, particularly in terms of some aspects of language processing. But is this always the case? Probably not, since dyslexia is determined by multiple factors. It could have to do with visual processing, or phonological processing, attentional problems, or a number of other underlying cognitive difficulties.
The field of learning disabilities is a fascinating one, and there are many aspects as to the nature of particular kinds of learning disabilities. Since there are so many aspects that go into what the underlying issues are in learning disorders, it is not surprising that extensive testing is usually needed to define a particular person's problem.