What causes the aggregate demand curve to shift?

Aggregate demand is the total demand for all goods and services in an economy; it is essentially gross domestic product. Its components are consumption, investment, government spending and net exports (exports - imports). A shift can be caused by a change in any of these components.
For example, an increase in government spending would cause the aggregate demand curve to shift to the right, which makes sense because you would expect this increased spending and demand in the economy to increase gross domestic product. Conversely, a reduction in net exports (through less exports or more imports) would cause a shift to the right.