Question

why could be the reason for melons and pumpkins cracking open while on the plant

Answer 1

It is a natural evolutionary mechanism of plants to ensure that their seeds are disbursed for the next generation. It is a part of the process called seed dispersal. All plants make an effort to ensure that their seeds end up in the soil where they can grow into a plant themselves. Some commonly seen mechanisms are :

1. Due to gravity - fruits are heavy and naturally fall on the ground (for eg. coconut)
2. Fruits develop into dry pods and then burst open to shoot the seeds in the air (for eg. Okra)
3. Fruits are aromatic to attract the birds and/animals who will disperse the seeds after consumption (for eg. Mango)
4. A combination of the above mechanisms

Melons grow on vines on the ground. So, they can't fall from a height and disperse the seeds. 
Melons are fleshy. So, they can't dry into a pod and shoot the seeds out. 
But they develop a very favourable aroma to attract animal/birds and crack open to expose the seeds to the dispersal agents thus ensuring that their progeny survive and develop into next generation.

Now, why don't all melons crack open and only some do? Because, most of the fruits or vegetables that are cultivated today are selectively bred to avoid 'cracking open' of the fruit for seed dispersion. We humans have altered the plants to suit our needs and not the plant's needs (though we do take care of planting the seeds and ensuring that their progeny survives).

Answer 2

Some commonly seen mechanisms are :

1. Due to gravity - fruits are heavy and naturally fall on the ground (for eg. coconut)
2. Fruits develop into dry pods and then burst open to shoot the seeds in the air (for eg. Okra)
3. Fruits are aromatic to attract the birds and/animals who will disperse the seeds after consumption (for eg. Mango)
4. A combination of the above mechanisms