Re: Regrading grabs in Aikido...
My experience as a woman is that when a larger man is trying to intimidate or harass you, rear grabbing attacks are pretty common. The attacker may grab your shoulders to stop you leaving, or to shake you; he may grab your wrists to pull your arms behind you or to keep you from reaching for a door, telephone, weapon, etc. If his hands are big he may intend to collect both of your wrists in one hand and then get up to mischief with the other hand. It's a different dynamic than two people fighting outright.
The point in my life where I really wish I had had access to aikido involved a back shoulder grab or bear hug (I'm not positive which) to stop me from running away, followed by turning me around and grabbing from the front. I was still thinking of the person as a social peer and was not prepared to knee him in the groin, but something non-injurious but emphatic like sankyo would have improved the situation. (In retrospect, so would the knee to the groin. But it's difficult to switch tracks fast enough to realize that someone is no longer an associate but an assailant.)
All of the back grabs I've seen have been from straight behind, either static or as part of a pursuit. I'm not sure where you would ever see the version where you start in front of nage and run round him.