I'm going with Zuko's Journey as a spirit quest.
As Zuko doesn't think ahead, he would get drawn into a trap against Fire Nation Soldiers and be cornered by Azula. His only chance, redirect lightning. Except, he lets it 'into his heart' and is near death. There he makes his way either being lead by or to his mother.
As to how she died
When she went to the fire lord, her husband (Zuko's father) was willing to give up his save for his own hide, but she said something to the effect of 'her life for her son's' and the Fire Lord took compassion on her, outraging??Zuko's Father, who??then??struck her down, killing her, and then killed his own Father who would have probably denounced him right there, for killing someone, he himself had just spared.
Since one royal death is suspicious, two might not be overlooked. So he gave a funeral for his father, and made it look like his wife just left.
It's just an idea, but I mean come on, Zuko's Father burned the face of his own son and banished him on??a pointless (at the time) quest. He's capable of anything.
Edited on 11/09/2006 10:39pm