A study by the American Academy of Pediatrics, the largest professional group of pediatricians in the United States, recently completed a study on the onset of puberty in boys. The article is available here on the AAP website. More research is needed to answer the "Why is this happening?" question. With girls, it is thought that some combination of health changes and nutrition have pushed the onset of puberty into earlier years compared to the past.
One thing this research does help is to provide us with an expected baseline for when it is "normal" for boys to start puberty. In an otherwise-healthy boy with the start of puberty at 10-11 years of age, we can now consider this in the normal range. Meaning it is not likely some bigger health issue is happening.