Why does adding lemon juice before baking cause a thick layer of dense cake to form?
The original recipe is leavened with baking powder, which these days is a mixture of baking soda, an acid, and a special acid which activates at high temperatures.

Lemon juice is strongly acidic, and will react with all the baking soda in the baking powder. This means you won't get as strong a leavening effect during baking. (Some people also feel that the un-reacted acid from the baking powder results in a metallic taste.)

To compensate for this, you can try adding baking soda (in addition to the baking powder) to the recipe; I've heard 1/2 tsp baking soda to 1 tbsp lemon juice, but I don't have any direct experience with that. I don't know what this will do to the overall leavening or taste of the cake.

It is unusual to see lemon cake recipes which use much lemon juice in the batter. Drizzling lemony syrup in afterwards is more common. You might want to try that instead.