Just wondering if anyone out there can tell me how  to avoid this.  

I put a filling or 2 (depending on the flavor of cake) in between my cake layers.  The last 2 cakes I have done have sat for a couple hours after being frosted and when I come back to them they have a HUGE bubble in the side.  I have been able to poke a hole in the bubble and then push the frosting with my hand or a knife back onto the cake.  Does anyone know why this is happening and how I can avoid it?  I really appreciate your help!!!!  THANK YOU!

It's hard to say why its happening, although there are endless theories.  Get a coffee stirrer (little things that are hollow) poke one end into the bubble and suck out the air.  Your mouth isn't on the cake and you're not blowing, you're sucking the air out.  Repair the little hole, and done.

After filling the cake, do you let it sit for a couple of hours before icing it?

I have the same problem, but it happens when I am in transport  and only on the small 6 inch cake.

Any suggestions would be appreciated.