Cracking is pretty simple. Low humidity causes the wood to shrink, and it shrinks more across the grain. The sides don't want to move in and out though, so the stresses are trying to tear the top and back wood apart along the grain line, where it is weakest to these forces. Similarly, when it becomes humid the top and back swell but the sides resist, so top and back will probably dome up, raising the action.
Repeat this cycle regularly, which happens most in the winter with central heating, and eventually the wood gives way.
Builders try to reduce this risk by using wood with a comparatively low humidity when the uke is glued up, doming tops and backs so they can sink before the stresses try to pul the wood apart, and so on. But there is a limit to what can be done.
If like me you live in a UK house with feeble heating and a benign climate outside, then you'll probably be OK if you don't store your uke next to a radiator (heat lowers humidity, and radiators switch on and off throughout the day, the worst possible case!). Of the ukes I've built, the only ones to crack were subject to cycles of high and low humidity.
If you're in the US with snow outside and 80 degrees of temperature inside, then your house is pretty dry unless you take steps to humidify your uke. Martins are no more immune to cracking than other brands because there's no more that Martin (or anyone else) can do to counteract your home environment.
On the bright side, cracks are a nice source of income for professional luthiers, so they'd in theory be happy for you not to humidify (except they all seem sad to see an instrument damaged that way).
If you don't want to humidify, assuming your environment needs it, then get a laminate topped uke or put up with cracks.