I don't think it's actually that many people's birthday. It's probably a date they just put in to conceal their actual birthday. They do it for privacy reasons or to hide the fact that they're underage. Don't you think it's kinda fishy that we have so many members all turning 29 today? Also, upon closer examination, I bet many of those 29 year olds are the same guy, who constantly uses some variation of "Jon" or "John" in his username. It would be even more of a gross coincidence if that many people have the same exact birthday, and all have that name in their username.
I could be wrong, but that's the way I see it.