Why are complementary goods defined as when a good's demand is inversely correlated with the price of another?
Why are complementary goods defined as when a good's demand is inversely correlated with the price of another?


We know that (in this model) the price of patties influences the demand of buns. This is because a cheaper patty makes the whole burger cheaper, which makes me demand more burger and therefore more bun . 


  Why would the demand of hamburger buns increase when the demand of
  hamburger patties decreased?


Other things being equal, the price of patties influences the price of buns. The theory says nothing about how the price change came about. You implicitly assume that something changing patty demand would change bun demand, also. In the model world, there are only explicit assumptions.