A contraction can be triggered by

1. The regular business cycle. Over allocation of capital to businesses and investments where there is over supply / under consumption. Where the market does not reward the creators of inferior products forcing them to cut spending or shut down production completely.
2. Defaults & Bankruptcies because financial institutions are over leveraged or where there is fraud.
3. Even just a slowing of growth rate of the debt currency in circulation relative to the ever increasing demand in an expansion can lead to a choke hold and a contraction.
4. External Shocks, Natural Disasters, War, Geo-political crisis or even just a perceived concern of them.
5. Psychological of the market participants, fear, overly greedy leading to unsound speculation
6. Political influence, Laws, Regulation, Tax changes
7. Technology, Inventions
8. Shortages, Price Shocks

and probably many more….