Of course in several chapters of the General Theory (especially # 12) Keynes discussed the vagaries of financial markets and how they can crash. But in general that is not the main mechanism for macro fluctuations in Keynes. While there were predecessors to Keynes who can be seen as emphasizing broader shifts in aggregate demand, including Malthus, Sismondi, and Marx, the more common model of macro fluctuations posited by classical economists of the nineteenth century often focused on investment declines after bank failures after the crash of a speculative bubble, with the Panic of 1837 and its subsequent recession in the US an example (due to a crash of a speculative bubble in cotton lands). I shall indulge by quoting at length John Stuart Mill on speculative bubbles, who certainly saw this as the main source of macro fluctuations in the manner described above (J.S. Mill, Principles of Political Economy, Book II, Chap. 9, Section 3, 1848):
"The inclination of the mercantile public to increase their demand for commodities by making use of all or much of their credit as a purchasing power depends on their expectation of profit. When there is a general impression that the price of some commodity is likely to rise from an extra demand, a short crop, obstruction to importation, or any other cause, there is a disposition among the dealers to increase their stocks in order to profit by the expected rise. This disposition tends in itself to produce the effect that it looks forward to - a rise of price; and, if the rise is considerable and progressive, other speculators are attracted, who, as long as the price has not begun to fall, are willing to believe that it will continue rising. These by further purchases, produce a further advance, and thus a rise in price, for which there were originally some rational grounds, is often heightened by merely speculative purchases, until it greatly exceeds what the original grounds will justify. After a time this begins to be perceived, the price ceases to rise, and the holders, thinking it time to realize their gains, are anxious to sell. The the price begins to decline, the holders rush into the market to avoid a still greater loss, and, few being willing to buy in a falling market, the price falls much more suddenly than it rose. Those who have bought at a higher price than reasonable calculation justified, and who have been overtaken by the revulsion before they had realized, are losers in proportion to the greatness of the fall and to the quantity of the commodity which they hold, or have bound themselves to pay for."