Maybe you're just unlucky? I keep my balls in my non-insulated, non-temp controlled garage. During the season I keep the balls in my trunk. Over the course of a year the thermometer out in the garage shows a min temp of 25 and high temp of 90+ and I haven't had any issues with balls cracking. I have balls sitting in plastic bags in the box, and balls just sitting out in ball cups. I do spend quite a bit of time out there rearranging, so they tend to get moved several times a year, and I do check them each time I move them - I think I'm just waiting for them to crack!
Some suggest it could be the drilling - dull bits, drilling too fast, not enough bevel, drilling too close to the pin, too much glue on inserts, etc?
I drill my own stuff and have drilled closer than 1" to pin, drilled half the pin out, followed the "minimum of 1 inch to the holes," and still haven't had any problems.
I've got original Break, Break Pearl, Hei$t, Big Hei$t, Lunatic, Hype Urethane, Break Out, Break S75 - all drilled and sitting out there. I also have numerous others from 900G/AMF that are undrilled and still no issues.