I don't know if this question has been answered cause there is 51 page to read.
Why some mix subbed in 2009 and in the 'to be posted' are still not posted?
There's an answer a few pages back, let me find it.
Though the bottom line is posting chronologically, there are things that sway which mixes are posted when. There's a lot of these, so it does kind of make it look random at times, but here's just a few examples:
1) An album mixflood
2) Some event happening that a TBP mix can be relevant to (see the Minecraft posting around Minecon, or the recent postings for Gario, Flexstyle, and Rozovian for their Workshop Mod promotion, or even the mini-mixflood on the Metroid anniversary day last year)
3) Outside material is released and some of it has been submitted to OCR (see The One-Ups Super Mario Kart mix released near the time they did their album, or the mixes posted when Hylian Lemon released his Essence of Lime album)
On the backend, there are some things that determine which mixes are available to post. Some of those older ones may have some minor thing that needs remedying, such as getting a proper bitrate version or consent of multiple artists if needed. There are a few other things, but you get the idea. It also really just depends on what Dave wants to post that day, but he does like to fall back on chronological order if all else fails.
Hope that helps answer your question!
Hopefully that doesn't step on the staff's toes.
I just know it's something I wondered about for the longest time, and made a point of noting when that answer was given.