As property owners the petite bourgeoisie benefit from capitalism, yet they are naturally disadvantaged in the capitalist system when compared to the haute bourgeoisie. The natural fate of the petite bourgeoisie in capitalism is to develop small business ventures that will then be bought out by bigger businesses at the expense of the small capitalists that own those small businesses. This is the competition of the "free market" in action. Socialism isn't the solution as they lose the privilege of bourgeois class status under socialism and so fascism was born as a solution whereby the liberal order established by the haute bourgeoisie is subverted & usurped so the petite bourgeoisie can seize control of the capitalist system.
Of course what actually ends up happening is that the haute bourgeoisie infiltrate and coop the burgeoning fascist movement when the threat of radical Leftism emerges and grows; this is why big capitalists always end up coming in to bolster fascist movements, spread their message, and finally take over in the role of leadership eventually pushing the petite bourgeoisie out of power in the very movement designed to empower them.
This happens because the the bourgeoisie as a class are motivated by individual profit and have no collective identity. Even if they're class conscious - and they often are - they rarely display an actual sense of class solidarity and the illusion of such is driven entirely by a desire to protect their own self-interest; not because they care about each other. Thus the bourgeoisie are just as likely to be at each other's throats as they are to unite in suppression of the proletariat.