Capitalism is prone to boom and bust cycles. Whether you choose to call this a failure, or the nature of the beast is a matter of opinion.
But, if you are preparing for a debate it is a reasonable place to start:
Why is capitalism prone to boom and bust cycles?...I am going to go with something derived from the Austrian school (but not entirely...)
Fluctuations in investment spending are the primary driver of economic / market volatility.
Investment decisions are informed by:
rational expectations, reactive expectations, fear and greed. Information is imperfect. With this combination we get environments where some investments look better than others. When sentiment changes it changes quickly. This is instability.
In the 2000's housing, (and housing related investments such as mortgages and mortgage derivatives) appeared to be both safe and profitable. Investment in these vehicles soared (you could say irrationally, but it seemed rational at the time). Soaring investment drove hiring in the construction and banking industries. When these vehicles started to appear less safe and less profitable, than previously imagined it triggered a massive reallocation in investment dollars.
Result is massive market losses, retirements destroyed, companies put out of business and job losses. In the capitalist system job losses in one sector have reverberations to other sectors of the economy.
I would also suggest looking up Hyman Minksy. He had some good (related) stuff as to why markets are inherently unstable.