That the burn acheived 97% of target impulse, based on the location of the debris. This is at the extreme range of variance for a solid rocket and suggests something probably did go wrong in the last few% of burn time.

Alex Blackwood has suggested that heat transfer to the on-board hydrazine tanks would have been a likely cause of catastrophic failure (explosion).

We may never know, but these appear likely scenarios. The next problem is that the troubled New Horizons Pluto-Kuiper probe is based on the same architecture and solid rocket. There may need to be a redesign, which will further delay NH!

All the above is a digest from Alex's Space board that doesn't allow casual browsers...