Sounds like a support problem to me, too. Use two or three cake boards, or some foam core, or a cake drum so that the cake doesn't bend when you move it.
Do you freeze your cakes?Are your cakes frozen or barley thawed out when you ice them? I had the same problem,being in a hurry I had iced my cakes barley thawed only to see the icing have small cracks after so many hours. Kinda like the ice over the ocean,it cracks and seperates in the warmer seasons.Cakes expand once thawed,causing the icing to seperate.Helped me out !
It could also be your cakes were not finished "settling" before you put the icing on.
Next time try this, might help........
Once you level, fill your cakes, cover in wrap, get a ceramic tile, fit to size and shape of cake, set tile ontop of covered wrapped cake, let it sit for about 1 -2 hours.
Then cover with cake crumbing, put in fridge to have icing crumb crust coat.
BEFORE you finish with icing, get a straight pin, poke some holes not very many holes in cake to help release any trapped air pockets.
It could also be, it's too warm in the room of where your decorating, too much shortening? Not enough powered sugar? Too much liquid?
HTH
I "think" if you look up Kitchen Krafts or SugarCrafts or maybe even google it, there is a "stabilizing powder" you can add to your BC recipe.
It helps to hold up the BC in High Humidity weather.
HTH
I just remembered that my recipe has a bit of cornstarch in it. I purchased name brand cornstarch when it was on sale and I had a coupon. My buttercream was too crusting and I had trouble getting it and keeping as smooth as I used to. I thought it was something I was doing. When I ran out the name brand and went back to the generic, the problem stopped.
Maybe this is your problem, too?
As far as I know all powder sugar has cornstarch in it. That is what keeps it from getting hard. 
Indy wouldn't agree, but my suggestion is to add a little more crisco. If it is dry, the crisco will help make it more creamy. You can also use piping gel.
How are you smoothing the icing? And when are the cracks forming?
How long after you ice are you smoothing it? When do you notice the cracks? Before you smooth, during, right after, hours later?
Okay well I was thinking that maybe you let it sit too long before smoothing it, but if it doesn't happen until hours after the smoothing then that isn't the issue. Hmmm.
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