One thing that can damage a steerer tube that few mechanics consider is the clamp used to guide the cut saw when it's shortened.
Take a spare length of carbon steer tube you've got laying around and clamp it up tight with a Park steer tube cut guide. If you use the force that might feel right from clamping a piece of wood that's going to be sawn, you'll see it will totally deform the thing.
Over clamping a steer tube when you're cutting it is a sure way to crush some of the carbon.