"Stacking" is a practice used by some boarding operations (kennels) where multiple pets, in multiple crates, are placed on
top of each other vertically and horizontally maximising the number of pets boarded at a time, in order to
maintain a low overhead and maximise profits. Usually the health of the pets are disregarded as hygiene is
poor, the environment is highly stressful, and daily exercise is not a priority. As a result, pets are crowded, the air is warm, ventilation may be poor, and it's easy for the spread of bacteria infections, resulting in "Kennel Cough".
Watch this video about Kennel Cough in Dogs