It's all fake.
Bet that got some attention.
Seriously, just take a brand new person who has never done aikido before and try irimi nage. Do they fall the same way that 5+ year students fall?
For most of aikido, there is a training paradigm where one learns specific ways to fall and roll. For training purposes, falling and rolling are, in effect, acted. There are various other ways to contort or move the body where you won't get those nice rolls and breakfalls. (Which, of course, means one other option is getting broken. Another option is kaeshi waza. Etc, etc.)
So, the question is really too general to ever cover any of the good info. More to the point is Why do people roll or fall the way they do in aikido? What are the causes? Historically, why do aikido people roll/fall that way? Are they taking advantage of the only "out" available or are they forced into it? Too much power forcing a redirect or just "synergizing" with nage?
As for nikkyo ... bah, unless you've got some internal structure (see other threads) going, nikkyo isn't really that hard to stop.