Citation :
704.7
This new rule was added for clarification surrounding a very popular question since undying was previewed: What happens if a creature with undying and a +1/+1 counter receives enough -1/-1 counters at once (say, from Black Sun's Zenith) to cause it to die? Will undying trigger or not? The answer: No, it doesn't trigger and the creature won't return.
State-based actions all happen simultaneously, so one moment there's a creature on the battlefield with +1/+1 and -1/-1 counters, and the next moment that creature is in the graveyard. Undying asks "did that creature have a +1/+1 counter on it?" When you look back to that moment before state-based actions were performed, you see that it did, so undying doesn't trigger.
This was derivable from the rules before, but the number of questions I was getting certainly indicated that it could be clearer, so now this new rule exists.