1) The canonical answer is: there was no before, the singularity happened at delta_t=0. It is a mathematical construct resulting from extrapolating a physical model backwards in time which somehow converges. I'm not sure its not just convention. The big bang didn't create space time. It
is space-time indeed and we know nothing (absolutely nothing, I believe!) about an object that has no spacial or temporal extend. All they are interested in is a few seconds after.
And the other big point is that our concept of time might be flawed. Maybe in relativistic terms, from our point of view it was a few seconds, but it actually took an eternity and you could keep observing closer and closer to the origin (in the diagrammatic sense) but never reach it, so who even cares, still? So, in a way more easily grasped by humans, it would be more apt to say, there is no after, because we will possibly never get behind it in our way forward. Does that seem sensible to you?
2) Because that's when the size of the universe in the calculation reaches plank scale? I'd like some correction because I have read before that plank scale is not actually a hard physical boundary, but I believe we just don't have the tools to say anything about that time with any certainty. And don't be mistaken, there are loads of competing theories and all have warts, paradoxes and open questions. Such is life, ironically, which I still think is a comfortable closure.