How do you put up a guard when you've been running on nothing more than adrenaline for 5 hours?
I don't know about you, but when I get physically wasted and feel like I can't stand, usually the arms are the first things that drop.
Seemed to me that when Luffy fell he couldn't even stand, but he did want to pick up Ace's card before leaving anyway.
You could also argue that Luffy believes Ace is many leagues above his own strength and holds somewhat of an admiration to his older brother. So when an admiral turns up, Luffy basically believed Ace would hold him off.
And on the other flip side, you could just say that Akainu is a bit faster than a slug and it went something like this:
1) Luffy falls
2) Akainu pounces on fallen Luffy
3) Ace gets fisted
Luffy may be fast, but he's not some dragonballZ saiyan with instant transmission. Akainu didn't trangress to some sort of inner monologue. It was basically a follow up attack on someone who had already fallen from fatigue.