average income - if the income increases that means the quantity demanded of a product will also increase hence leading to a shift in the demand curve as people will have more disposable income, hence able to purchase more items
seasonality- a product may experience a shift in the demand curve as a result of the season, people tend to purchase a product in a certain season . so when the season ends , the demand for the product reduces, hence the quantity purchased decreases.
availability of substitutes- a shift may be experienced as a result of availability of substitutes int he market. if a product was enjoying monopoly then a substitute is introduced, the product will experience competition from the substitute as people will have the freedom to choose which item to go for (quality, price etc)