what causes increases or decreases in aggregate demand
All of these factors are macroeconomic factors that may increase the aggregate demand. Monetary policies cause increases in aggregate demand because the central bank of a country uses this particular economic factor as a tool to manipulate the spending of the citizens in a country.
If the federal reserve raises interest rates, then we will see aggregate demand decrease or shift left because it has become more expensive to finance investment. Alternatively, if the federal reserve decreases interest rates, we will see investment increase, and aggregate demand will shift right.