Bread-Bakers v107.n038.4

If you dust your loaves with flour when you're forming them, the 
flour could be forming fault lines within the crumb, and the bread is 
breaking up along those faults.

I think that a lack of structure in the crumb can also be due to too 
little protein in the flour (I like KA's unbleached bread flour, 
which is 13% protein) and/or not kneading it enough, to build the 
strong gluten.

--Chris