When you look on the inside of the molt, do you notice any evidence of mold? That could indicate that there wasn't enough airflow and the T was taken by mold. Or it could have been a lack of humidity/dehydration.

Avicularia spiderlings are notoriously hard to keep, and have been known to just up and die for no apparent reason sometimes. Don't beat yourself up about it, sometimes that's just the way it stacks up.