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This old common question is still not clear to me. I got this explanation:

Bananas go through a unique process known as negative geotropism. Instead of continuing to grow towards the ground, they start to turn towards the sun. The fruit grows against gravity, giving the banana its familiar curved shape.

But why? The answer lies in the botanical history of the banana. It originated in the middle layer of the rainforest, where there is little sunlight. If the fruit were to grow towards the small amount of light that penetrates sideways through the vegetation, the plant could overbalance and topple over.

So bananas developed a way of growing towards the light without destabilising the plant.

source

So, evolutionarily, the bananas are 'forced' to grow towards the light. But how is this capabilty 'grown into' the banana? Is it something epigenetic or is it just light that attracts them? So how does this mechanism works?

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    $\begingroup$ The last paragraph makes the question a little unclear. Are you asking "why" in the sense of "what are the evolutionary forces that pushed the banana to grow like that?" or in the sense "what are the physiological mechanisms that causes the banana to like that?"? $\endgroup$
    – Remi.b
    Commented Aug 5, 2016 at 20:12
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    $\begingroup$ Note by the way, this is what a wild banana look like. $\endgroup$
    – Remi.b
    Commented Aug 5, 2016 at 20:15
  • $\begingroup$ Note that this curved growth is hardly unique to bananas. Cucumbers, some kinds of chili peppers, pea & bean pods, and many others can all have similar shapes. $\endgroup$
    – jamesqf
    Commented Dec 4, 2016 at 6:15

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The "proximate cause" would be a differential rate of growth on the different sides of the individual bananas. If the cells on the underside of the banana are dividing faster than those on the top, that will cause the upward curvature.

As to what causes the differential growth: I'm not a banana biologist, but I would suspect that a growth hormone of some sort finds itself more highly expressed in the part of the fruit that starts out closest to the stem.

Why that hormone might have decided to start doing that, and what selective advantage it might have given the early wild bananas, I have no idea. But our modern cultivated bananas have probably been strongly encouraged to do it, for compactness of shipping, if nothing else!

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