Order of combining wet and dry ingredients when baking
The dry ingredients are lighter, less dense, and less viscous than the wet, which means they'll have a tendency to float on top of the wet. The dry ingredients that come in contact with the wet will form a sort-of skin, preventing the wet from distributing throughout the dry. You end up with clumps of dry, unmixed, and (ultimately) uncooked dry ingredients.

Pouring wet into dry forces agitation and more uniform distribution.