There were plenty of times in the game where Adam could have gone down in order to make more of the situation....when he was slapped in the face and elbowed in the ribs are two that spring to mind....but he didn't.
I think his 'dive' was born out of sheer frustration on several fronts.....not getting anything for the aforementioned Pompey misdemeanours, the fact that he had run himself into the ground for the cause, the fact that he had come mighty close with some fine efforts on goal, the fact he was probably thinking he should have gone down upon the initial contact.
I think his 'dive' was just a culmination of events and that he was simple desperate to see his beloved Saints win the game and what it meant to the fans.
I'll take Adam's total passion for Saints,if that comes with the odd 'dive' every now and then, any day of the week.