Dream of stack
A stack could be a stack (pile) of anything. In this picture the dreamer saw a fierce dog and fell down in fright. She fell backwards and knocked down her friend, who also fell backwards. The two people looked like they were stacked one on top of the other.
Dreaming of stacking
A dream about stacking is an unusual dream. The dreamer may have a situation in real life, where stacking is a theme. The dreamer may have used stacking and this activity stuck in the mind. At night, sleep frees the mind from other input data and the brain recalls this memory of stacking to make a dream. The dreamer should be able to identify which activities in real life were represented by the metaphor of stacking.