I would guess that from his perspective (which may not be what the court decides is the right perspective, of course) he might be thinking something like, "hey, we started this together, I had these 10 ideas, we parted ways because we have the wrong personalities to work together, now I see 8 of my 10 ideas in the finished product (even if it's out of, say, 100 total ideas or whatever) -- and I feel like you never actually compensated me for about a month of genius that I provided. By helping you start this thing it was secretly our baby all along, and I was okay with you doing whatever you pleased with it while it was a bunch of crazy ideas that hadn't proven their mettle; but now that they have, this is my last chance to speak up for my fair share."

Again, that's not necessarily the truth, but if you want to understand the psychology of how someone can claim this stuff and feel reasonable about it, that's how. We've very much only heard one side of this discussion and while Altman's side is very reasonable, it'd be interesting to see the other side.