Stress cracks can happen for a number of reasons. Curve not matching the body is one, but usually the least likely, because the glass is placed onto glue between the body and glass. The glue is not a liquid, but it's not solid either (obviously until it dries). It's a polyurethane type bead. Because of it's consistency, you can have it sit flatter in parts than others, negating the difference in curvature etc. Unless the glass is viciously distorted, so much so that it simply does not fit.
Other causes are faulty glass (weak point that can crack, or what's called a "shell" which is like a stone chip, but in the edge of the glass - which can crack with vibration and twist from the chassis moving while driving). Can also happen from pressure against the glass, such as a scuttle being put back too tightly - obviously this depends on the car; I've seen it happen a bunch of times to certain Mercs that have a plastic scuttle that sits against the screen, and is held in place with screws that pass through the scuttle to the bulkhead below the screen. Over tightening of the screws can, and does, crack the screen.
Incorrect fitting can also crack screens - not enough glue, too much glue, incorrect position of the car during the placement (ideally, you want it on flat ground where all 4 wheels are at the same height. Gentle slopes aren't a problem, but having one, two, or three wheels on a kerb or uneven drive can allow the car to "sag" while the screen is out, then the new screen is bonded in it sets in that position - then you pull off the kerb or drive and drive it, and it can crack. Had this happen to me back in the early days, with a Ford Galaxy. Now I insist on moving the car if the position isn't ideal.
Foreign bodies such as spacer blocks, or grit in the glue, can also crack it. Many cars use rubber spacer blocks under the glass and along the glue line to set the height of the screen, but I've seen them crack glass before too. Had a Volvo S40 in a few years ago that had stresscracked after fitting, only to find the fitter had put runner spacer blocks along the bottom of the screen, and because they weren't needed on that model, the screen had crcked against one. Likewise, NOT putting spacer blocks where there should be ones can also lead to cracking.