What is the most common reason for a galaxy developing an Irregular shape?

Severe distortion from gravitational interactions or collisions with other galaxies

Irregular galaxies are defined by their lack of defined, symmetric structure, which stands in direct contrast to the organized forms of spirals and ellipticals. The primary mechanism driving a galaxy into this chaotic state is external disturbance. When galaxies interact gravitationally, especially during close passes or outright collisions, the mutual gravitational pull can stretch and distort the component parts—stars, gas, and dust—creating elongated structures known as tidal tails. This process scrambles any pre-existing orderly structure, resulting in the characteristic irregular appearance, often seen in smaller galaxies like the Magellanic Clouds orbiting larger neighbors.