Substituting buttermilk powder for buttermilk
Cakes are chemically leavened with baking powder or baking soda. They have a narrow pH range in which they work. 

Pineapple juice is very acidic. It probably reacted with your leavening and deactivated it. 

You can use buttermilk powder for the cake, but you have to mix it with water, not fruit juice. Then it will work as usual.