Maybe they mistook the command and thought he said Parade Detention, not Attention...
I bet it was a sight to see, suprised as said they never simply moved everyone fwd and carried on without them, still think 30 days and out would have been fairer.
They were rather foolish full stop, thinking about it they could have simply screwed the parade up without dropping themselves in it, a miss understood command and marching on as the others turn etc or ticktocking etc would soon. Indeed there was always a way to make those that you feely wronged by look a Cnut.
I knew someone on my old Squadron charged for something petty, he was awarded a derisory punishment and sniggered at it, he was marched out, charged and marched back in for sniggering, he simply said he tried to contain a Sneeze in his defence, at the will you accept my punishment he elected for court martial..... Charge dropped.