I don't doubt that this happened, and I have a couple of theories why:
(1) The "last days". When I was a JW, I saw doctrine adjusted (i.e. "new light") to change the qualifications of the "anointed" to those who were able to comprehend the arrival of Christ's kingdom in 1914. As far as I know, the JWs (for the time being, anyway) still adhere to the belief that Armageddon will occur before the last of these "anointed" ones passes away. By continually reducing the age for baptism, they in effect allow for more elderly individuals to claim their "anointedness", through an unspoken statement that comprehension of these events could be understood as young as age five. When I was a JW in the 1970s, it was understood that children did not need to be baptized, because they would be under the "protective umbrella" of their faithful parent(s) at Armageddon. Apparently, this is no longer the case.
(2) Numbers. The WTBTS continues to make assertions that their numbers continue to rise worldwide, when, in fact, in most western countries, the numbers are flat or, in some cases, declining. The easiest way to bolster these figures would be to add to the ranks of the baptized by allowing young children baptism, even though they are unaware of the ramifications this will have on them later in life. The poor kids are, I'm sure, doing it to please Mommy and Daddy, not Jehovah.