It is simply the contradictions inherent in the system becoming extremely heightened and straining towards their resolution (aka socialism). Here’s a passage from Fundamentals of Marxism-Leninism on the decay of capitalism under imperialism (its final stage of development):

When capitalism reaches the stage of imperialism, it inevitably enters the period of its general crisis. Capitalism experiences periodic economic crises, a defect organically inherent in this system. However, the general crisis differs from these in that it is an all-embracing crisis of capitalism as a social system. It is a permanent state and is characterised by the progressive disintegration of capitalism, the weakening of all its inner strength—economic, political, and ideological. The general crisis is not accidental, a quirk of history, or the result of mistakes by bourgeois leaders, but the inevitable and normal state of capitalism in the epoch of its decline and disintegration.

Under conditions of the general crisis of capitalism, this system is no longer able to keep peoples in subjugation, and one after another they throw off the yoke of capital and take the path leading to socialism. That is why the period of the general crisis of capitalism is the period of its downfall and replacement by socialism, the period when socialist revolutions and national-liberation movements against imperialism develop.

Ideologists of imperialism believe that if the victory of socialist revolutions could be prevented and the communist movement suppressed, capitalism would be able to remain firm and stable and prove itself the only possible form of society. They see the source of capitalism’s troubles solely in the action of forces outside the capitalist system. Even those of them who recognise the general crisis of capitalism as a fact seek to attribute this crisis to the existence of the socialist system and to communist plots to overthrow capitalism. The communist movement, which inevitably develops from the class struggle, is regarded by them as a movement inspired from without and organised by “foreign agents”. Actually, the general crisis of capitalism is the product of the internal contra- dictions of imperialism. It becomes sharper and deeper primarily through the action of capitalist society’s own antagonisms. External conditions—the existence and growth of the socialist system—promote the more rapid maturing of these antagonisms, but are by no means their initial cause.