MLB did a study/survey that showed the vast majority of major league players were born in the months of August through November.
Maybe it all goes back to Little League where league age is August 1 through July 31. So if you're an August, September, October, November birthday you're the (relatively) oldest and often biggest kid in each particular group.
A kid born 8-1 is in the same group as a kid born (the next) 7-31, yet is truly a year older, so the late spring, june july birthdays have become viewed as "bad baseball/sports birthdays".
It all seems or should even out over time but, the MLB stats apparently show that Aug-Nov birthdays give you the best shot.....
Just for the heck of it, just looked up the BoSox and Marlin rosters. On the Sox, 10 of 25 are born august through november and on the Marlins 11 of 25 are born august through november so nearly half of both teams are born in those 4 months (the 'good' Little league birthdays).