Failure isn't a correct term - and while 5/6 attacks is unlikely, it's not astounding. In any case, the chance depends in part on the composition of the party. The way random targetting works is the actor is added to the array (4 - position) times. Front is 0, so that actor is added to the array four times. Rear is 2, so that actor is added twice.
Thus, there is a 1/3 chance that the rear actor would be targetted by random targetting. So it's unlikely, but not impossible. Also, it is not linear with the number of people in the party. In a party with one front character and three rear characters, the chance that a specific rear character will be hit is 1/5. In a party with three front characters and one rear character, the chance the rear character would be hit is 1/7. And in a party with only one front character and one rear character, the chance is 1/3, which isn't that unlikely.
One thing to note, a sample size of six is not a reliable sample size.