I baked, torted and cut out the center of the cake as instructions indicated. Once I set the top tier on top of the bottom tier it all looked good. Then the next morning the bottom tier began to crack through the fondant. I tried to patch up the crack but the crack led to that whole side of the cake falling over. 
Was this due to the cake mix not being dense enough or does the angled side of the cake have something to do with it.
Well, I had this happen to me when using a marble cake mix made following the box instructions. So I would have to say that yes, if the cake isn't dense enough, it will crack. I had even double boarded all of the tiers and dowelled them correctly and still the centre tier of the three tiered cake cracked at the dowels. I had also used centre dowels all the way through the cakes.
So most definitely go with a more dense cake, you can dense up a cake mix by really reducing the oil added and changing it to be more of a pound type of cake with a firmer texture.
Hugs Squirrelly Cakes
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