Question, About the yellow flag?

[MaTH716]

I am a casual Nascar fan. I enjoy watching it. I know most of the drivers and teams and I don't watch it for the crashes. Can someone explain why the yellow flag did not come out right away on the final lap of the 500. If the rule is that once there is a wreck, the field freezes then what happened? Is that even the rule? Because if it is, how can they change it on the fly of the biggest race of the year? Or is it like one of those NHL rules, where a trip isn't a trip with 2:00 minutes left in overtime? Personally I liked it the way it was, when they did race back to the line. Can someone clear this up to me.