Re: Origin of ushiro ryote and ryokata dori
Originally, they were frontal attacks, where you "yank" the person around and get behind them. Quite common in older jujutsu styles.
It's a fair speculation that in Daito-ryu/aikido, these techniques were integrated as a way to put 'stress' on the body from behind - can one retain a connected body while behind pulled in some direction from behind.
This is a logical supposition because if there were purely a concern for combative readiness from behind, there are a number of more effective ways to take a person downwards or off-center from the rear that no DR or aikido system considers. Therefore, it's very likely that they adapted elements of older combative forms and reworked them as a training device to develop the 'aiki body' from a particular 'angle.'