Basically this - or as I'd like to call it, 'just freaking let myself be distracted'
I think it's a good thing though; I'm glad I'm getting ideas for my future projects this early on because the main one is a very lore-heavy series that kind of needs years of behind-the-scenes development for it to really feel alive; it wouldn't be possible for me to pull that amount of lore out of my backside on the spot when I actually intend to start writing/drawing it 
My ideas don't all take place in the same universe, but the vast majority of them do fall into my 2 main ones and I totally feel like that eases the pain quite a bit, though possibly for different reasons than you XD It makes me feel like I'm not scattering my attention into a billion different projects, but rather building upon my existing projects, creating depth rather than breadth 
Ye I basically don't think of reading order experience; I figure that if I'm going to complete them all anyway, they're all going to exist and at that point, readers can read them in whatever order they like (and I can recommend the optimal reading order after the fact :P)
Besides that, my brain would probably draw me into a particular order of creation anyway; if I actually make a deliberate choice to create story X before story Y, that's just tempting fate and making my brain want to do story Y first just to be difficult XD