> "...where does all the energy come from that provides the angular momentum of pieces rising to the higher orbital distances?"
The video states it's a fragmentation event with delta-v's up to 2 kilometers per second, a genuine explosion. That's enough to raise fragments quite high, but they still have to come down and pass thru the fragmentation point.
Other fragments are driven down as well, but due to the steeper gravitational gradient they don't go as far down as the high fragments go up. I think that's why they selected the 900km orbit altitude, so that no part of the pattern actually touches the Earth.
It also states that this sim is confined to the original orbital plane, but in a real event most of the fragments would not remain in the orbital plane. That would be much harder to sim, and I doubt it would really differ that much from the 2-D sim. All the fragments would still return to the event point anyway. Not all at the same time, of course.