Do We Ever Find Out How?

[Buffyverse Fan]

and there was magically a portal back to sunnydale, that happened to be right where he came in? i dunno, i think something or someone had to make that portal open. we learn in the buffyverse, it usually takes rather strong magicks to open a portal to another dimension, they don't just exist... so i think yeah, either PTB or Wolfram & Hart. both make sense.

Well Clem's point isn't as ridiculous as you make it sound. Both Connor & Holtz did just that-fought their ways out of Hell (probably by defeating some being that could open the portal).

That being said, I don't think that's how it happened. It could have been the First (it wanted Angelus back in the mix), Wolfram & Hart (they want him on their side in the Apocalypse), the Powers That Be (they needed their Champion), or the Power That Was [Jasmine] (She needed Angel to help birth her and be her Champion).