What is root cause?

I do not think the cause is the root.

I see a big empty block, what is it meant to be?

Uhm, maturing at small size? I wonder if it's too hot or too much sun?

Other than the temperature guavalane mentioned, could there be a change in pollination pattern? For example, sudden lack of male flowers blooming at the same time or bees?

It's also possible this is due to soil nutrition and moisture.

One of the reasons I think is excessive heat and at the same time scarcity of water.

gardendawgie,
it seems colhan is looking for main reason for melons ripening too young.

Few of bitter melons in my garden are yellowing too but at the earlier time when they are small in size.
Pic. of yellow bitter melon.{{gwi:105407}}

I believe this may be due to lack of pollinators as a result of excessive heat and lack of regular watering.I believe it is not due to early ripening of the fruit.

Same is happening to bitter melons in my front yard. There are like dozens of them growing but they stop at a fingerlength and turn yellow finally decay away.

Although poor pollination is a possible cause, IMO the most likely cause is either nutrient depletion, or some form of stress... such as temperature extremes, or water stress. What are the growing conditions like in your location?