What causes your fondant to crack and blow out? and how do you prevent this from happening?

This is usually because the fondant was applied to a cake that was too cold or not allowed enough time to settle. Air and gas try to escape as the cake settles and comes to room temperature. I start with frozen cakes so after I fill and crumbcoat I let my cake settle on the work table for at least 3 hours to come up to room temp and settle. Then I smooth out the buttercream to smooth out any filling that might have splooshed out during the settling process and sort of uncrust the buttercream so the fondant will adhere. The only time I've had blowouts or bubbles is when I have rushed this process.

Cat

i only get this when I use buttercream under fondant, for the reason above that cat described.
I use ganache typically, and do not have this problem. Much firmer medium to work with IMO.
Also have to make sure that the fondant completely adheres to the surface. Inspect closely for air bubbles and smooth out to avoid blowouts.