It seems like it may have been hinted at in one of the flashbacks (there were a few of them in the last episode) and it always seemed like it had something to do with that tree that Binchou-tan's always looking at (and writes a letter at the end).
Did her grandmother die at the tree? Did she die of old age or was it sudden (like getting attacked by a wolf)? It seems weird that she'd abandon Binchou-tan in the middle of the woods if she knew she was dying.
