wrappers seem to split from humidity shifts, from my experience. sometimes it depends on where your humidifier is located in your humidor, as to which cigars split.
in my 150 ct humi, the humidifiers are located on the back of the drawers, and sometimes after i have refilled them, it seems the cigars closest to them take in more humidity, and split when they lose it. usually, only at the feet, as thats the closest part to the humidifiers.
just my .02