I'm not sure why this happens, but I do have an attempt at a solution...I make sure that my cake board is VERY sturdy to that it does not bend when the cake is picked up. I've also resorted to using sanding sugars or sprinkles if the cake is simple and has cracks...it seems to hide all of the flaws. I'll be watching for help on this area as well...so thanks!
Sounds a like cake board sturdiness issue. I use at least 2 cake boards for any cake 11x15 and at least 3 for 12x18 or larger. If I have to cut my own boards, I turn the grain of the boards in opposite directions to add strength.
This may be a silly question, but how do you pick up the cake? I'm sure you're aware, but I see a lot of cake civilians pick up a cake by holding the two sides. When they come to the shop to pick up a cake, I practically fly across the counter with a loud, "NO!!!!!!!!!!!!!!!!!!!!!! NOt like that!!!!!!!!!!!!" and show them how to slide BOTH of their hands under the box, centering their hands in the two halves of the cake, to give proper support.
Definitely what they ^^^^^^^ said!
Now, if making your board(s) stronger still results in some cracking they you need to make your icing a bit creamier. You don't want to thin it w/liquid, but add a bit more fat (Crisco or butter, whatever you use). You will have to play around with how much....it might be as little as another Tablespoon, but most likely up to as much as 1/4 - 1/2 Cup 
I share in your frustration!! 
I have heard of people using a "few drops" of white vinegar in their buttercream to prevent cracking. Does anyone do this and how many is a few? I always doulble Wiltons recipe = approx. 8 cups.
I agree with the others....it's the board. I had three weddings this weekend and 2 were great...but the last one I did not have a heavy wooden or masonite board so I wrapped a base of 5...16inch cardboard rounds for a 14 inch base cake and I noticed that, that base had the crackles but the others were fine. For me lesson learned...
) I put a little vinegar in my icing, and I started using the Hi ratio shortening recipe and it's way better.It don't crack at all.
I put a little vinegar in my icing, and I started using the Hi ratio shortening recipe and it's way better.It don't crack at all.
Duplicate post, please also see:
http://www.cakecentral.com/cake-decorating-ftopict-639999-.html
HTH
I've added vinegar to my icing... I think I added a 1/4 tsp to a double batch (4 lbs sugar) of icing. You can probably add up to 1/2 tsp, but it doesn't affect the flavor at all. I've noticed that I don't have cracking if I use whipping cream for my liquid and make sure that it is "creamy" and not too thick.
1/4 inch foam board is much sturdier... Little flexing and you don't have to worry about doubling up or the grain, which can be tough on the bigger pieces.
It may be a support problem but I never use cardboard. I always use masonite boards covered with white contact paper. They do not flex, so it would not be a support problem. I still have cracks. It makes sense that the icing needs to be more creamy. Next time I'll try to vinegar and more crisco.
Pulled my cake out of the fridge this morning after smoothing the buttercream to perfection last night. It looked like a 90 year old woman. So it that completely the board support or is the cold affecting it as well? I can definitely get stronger boards and I might try to vinegar solution too, but want to make sure I'm not doing something wrong by refrigerating. BTW - I do not cover it in the fridge.
You are NOT doing anything wrong by refrigerating a b'cream cake :) The cold has nothing to do with what you experienced. It definitely sounds like your base board is not strong enough. You don't tell us what size the cake or what you have it on but that's my best guess.
Other possibilities: Your icing is too thin and runny OR it is too stiff - needs to be creamier. Try another icing recipe. This is a great one: http://cakecentral.com/a/2-icing
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