Ethylene

I came across this by accident. But it was interesting reading. I always wondered about this, have recently been affected by this.

"Bananas use ethylene to stimulate fruit ripening. A bunch of bananas will stay green for a long time until the ethylene concentration in the air around them becomes high enough. When that happens, then they begin ripening and they begin releasing more ethylene, which makes them ripen faster and release more ethylene, etc. In this way, all of the bananas will ripen very suddenly and simultaneously." - Ask a Scientist 

 It has both positive and negative effects. The negative is obvious that one bad (ripe or overripe) fruit can quickly damage the rest. The positive is that you can quickly ripen a bunch, or hasten the process, by storing them together in an enclosed environment. No wonder then, that my granny used to store mangoes inside bins storing rice to ripen faster.