How does celiac disease cause nutrient absorption?

Harms sm. intestine. Celiac disease leads to inflammation and damage of the lining of the small intestine, which causes decreased absorption of nutrients, vitamins, iron, etc... The intestinal lining loses its microscopic "villi", which are tiny protrusions that maximize the absorptive surface area needed to absorb nutrients. A patient gets diarrhea, anemia, lactose intolerance, tummy aches, irritable bowels, etc...
Celiac. You mean malabsorption. Basically the IgA antibodies in your system attack the cells that produce the enzymes necessary to break down gluten products. You are unable to absorb effectively and have the GI symptoms.