jumsumtak
Some of the reasons have been already mentioned. Also, we may assume people on this forum tend to be the ones who start early, or clearly, are extra motivated, putting in the extra effort to submit their apps early. However, I would wait to see R2 numbers post the decision date. btw, have you looked at previous year's threads? That should give a better picture.
Part of the reason I was asking is because I have looked at older threads. But maybe you're right and I should wait until Round 2 to get any sense.
Here's a picture by gbrooks6687:

(Maybe this wasn't the best to link since Tuck has EA which screws with things...)
From what little I know about the bschool app process, nowadays it seems like more people know they want to pursue bschool months/years in advance, while this may not have been true traditionally. Maybe partly because bschool has become more competitive through the years. If my assumption is true, it would only make sense most people would apply in Round 1 - especially for all the reapplicants, since there is the most amount of time between R3 from last year until R1 of this year.
I'm just rambling random thoughts while I wait a few months for decisions to come back....