The cause of that union is ignorance.

After explaining the purpose of saṃyoga, Patañjali reveals its cause: avidyā, fundamental ignorance.

This ignorance is not intellectual. It is a primordial erroneous perception that precedes all conscious experience.

Why does puruṣa, being pure and free, appear to be bound? Because of avidyā. Why does it confuse the impermanent with the eternal, the phenomenal with the real? Because of avidyā.

This sutra closes the circle: ignorance causes union; union allows experience; experience, when rightly directed, generates discrimination; discrimination dissolves ignorance.

The cause contains the seed of its own dissolution.