I think it went something like, 'Shit we need to record a lot of new dialog, but charging more than $40 for DLC is going to hit a nerve. What do we do? ... Nolan North? Done."I wonder if they wanted Dinklage back for new dialogue but he said "fuck you" so then Bungie said "no, fuck you! and just decided to replace him entirely.
Mass Effect solved the problem by using real VA for main characters and famous VA for the side stuff. With that method their main VA was put to work on a lot of side projects while recording mainline stuff. Mark Meer was used for Volusseseisisisisi and Vorcha to great effect.