It's about trading off between having new offspring, and investing in current offspring. I'd hazard a guess that it's more likely to evolve in cooperatively breeding groups with long lifespans....
Article on BBC
Read more about the "Grandmother Hypothesis"
The "grandmother hypothesis" is a great example of indirect/inclusive fitness. This behavior is exhibited in many species, including humans. The relatedness between grandmother and their "grandchildren" supports that after the factor of direct fitness is over (ie menopause), they can use their resources to help spread their genes. It makes sense why grandmothers live long after they are reproductively functional.
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