American Academy of Paediatrics
A reputable body and their paper should be taken seriously.
Onset of puberty in both sexes brought forward by between 6 months and two years. If in the majority of cases it is 6 months or a year, then this is not "earth shattering" finding, but if the majority were 2 years forward, then this might be significant. Their definition of puberty was mainly that of breast development in girls and development of pubic hair and testicular enlargement in males.
I would have preferred the onset of the menses in girls as an indication of the onset of puberty.
If we "assume" that the majority of onsets were 2yrs premature to previous decades, then I have no answer.
In girls my bet would be the indolent life style and obesity leading to an increase in female hormones......just a guess.
In boys, I have no idea as obesity is less connected to hormone increase than in girls.