From the wikia page,
This battle ended in the death of Bellatrix, who continued laughing derisively at the efforts of her opponent until the moment she realised Molly had defeated her.
Hence, it can be clearly inferred that Bellatrix treated Molly as a weak opponent and her overconfidence led her to death. On the other hand, Molly was highly angered as Bellatrix was torturing Ginny at that moment and the motherly love gave her the courage to face a death eater.
I can see only psychological reasons behind Bellatrix's death as it was difficult to overpower her when she was in a sane state of mind.
An extract from the wiki page:
... fires a curse that hits Bellatrix right over the heart, killing her.
Though the exact curse used is unknown, I believe she used a legal curse(such as the Stunning curse) and the placement directly over the heart is what killed Bella.
If you remember back to OotP, someone (perhaps Madam Pomfrey) remarks that it was a wonder that McGonagall did not die from so many Stunning spells. So, perhaps it is possible for one to die from a well placed Stupefy or some other type of Stunning Spell, and this is what I believe happens here.