[VadeRetro]

“We propose that the universe, before expansion, was an unstable brane that decayed into innumerable string-loops to form the universe as we now know it.”

Yes, yes! But that's the trivial part. Why was the brane unstable? Was this your brane on drugs? Creationism?

[RightWhale]

Why was the brane unstable?

Better ask how rather than why. But, either way, if it was unstable it must have been unstable before. This approach will be fruitless since there would have been no particular time for the nut to explode and it would have happened much longer ago than 14 billion years. In fact, it would have happened an infinity of time ago.