Why do watermelons split?

It seems it has to do with how much water the watermelon has soaked in a given time interval:

http://who-what-where-when-how-why.blogspot.com/2011/09/why-do-watermelons-crack-split-open.html

And some types of watermelons are thin-skinned and have a higher propension to split easily.

The article link above states that it might also have to do with pesticides and fertilizers, but I doubt it applies to watermelons here in North America.