It's mostly to be found in the comment sections of sites and/or journo's posting some inside information about it in their articles...
So no actual information then, just random comments from supposed ex-valve/origin employees.
I have no doubt that the new DLC policy didn't sit well with EA, as they have been building themselves around the DLC model more and more with each game. What I do doubt is that that policy is the reason for 'Origin', especially with how it came out at around exactly the same time. Stuff like that isn't usually done on the fly.
They also had a whole plan of using the popularity of BF3 and Star Wars TOR to promote Origin as a store.
I think the problems between EA and Valve are just as much over DLC as they are over Origin though. I highly doubt Valve is interested in helping their competitor's store set up by selling games that have to be installed through that store. (Ironic as it may seem coming from Valve but it also makes perfect sense that they're not going to be interested in helping another retailer do the same.)
Look at Battlefield 3 for instance. It has no ingame store, and it's DLC are sold separately in digital stores. Yet it's not on Steam either. This isn't an issue of DLC anymore. It's that Origin is pushing through their own digital retailing platform, and Steam doesn't want to aid them in doing so (most likely). The fact that they said in the Origin FAQ that most new games from EA will require Origin to be installed isn't helping either no doubt. They've basically decided to go with their own service. At this point all we can hope is that either one dies out or they push each other into delivering a better service, or I guess ideally, allow each other to sell the others games and DLC regardless of their own DRM schemes.