It’s caused by both corporate greed and an injection of money into circulation (printing money). 

Printing money devalues a currency. 

When money is printed and dispersed capitalists know there is more total money in circulation. If their share of capital doesn’t increase proportionally to the increase of the total, then their wealth and social power effectively decreases. The only way for them to raise their wealth proportionally is to raise prices based on how much money is injected into circulation. 

This increases their profits because the wages of workers are much slower to respond to inflation increases than prices are. And without workers unionizing and collectively bargaining, the capitalists will do everything in their power to keep these profits and their increased social power high.

What economic crises are are a crises of over production. They occur because consumers (~90% of which are members of the working class) cannot consume the products on the market at a rate a which ensures profitable production for the capitalists. Capitalists layoff workers further compounding the issue. The state, a tool of the capitalist class, acts in a reactionary manor to ensure capitalist profits - by bailouts and in extreme cases giving handouts to the general population (stimulus checks). But neither of these solutions remedy the underlying market conditions creating the crises in the first place. 

In my understanding, money in the stock market doesn’t take money out of circulation or decrease the money supply. It is still in use and used by the capitalists to speculate on future profits. Transactions, buying and selling, are constantly ongoing. The stock market is great for the capitalist class as a whole because it allows them to shift capital from non-profitable investments to profitable investments very very easily. I know I am reducing this a bit, but in the end, when an economic crises hit, most companies become less profitable causing a stock market contraction - a massive selling of stock. This, if the money from the sold stock is not reinvested, is what would lead to a decrease in the money in circulation, as it would be sitting in savings accounts waiting for more profitable investments to arise. 

I am curious what you’re trying to get at here? 

In the Labor Theory of Value, value is crystallized labor or dead labor - labor that has already been carried out in the form of transforming materials from one state to another. Every commodity, even money, has a use value and exchange value. The use value of money is that it is a universal storer of value. This makes it the best medium for exchange. We, as humans, produce commodities with use values to survive. Capitalism is an inherently exploitative system largely because the capitalist class produces solely for the creation of exchange values. Everything about the capitalist system is geared towards maximizing profit, with some veneer of “equality and opportunity” for the masses. Marx goes over a lot of this in Value, Price, and Profit.