Borrowings
Some characters 'borrow' their form from other similar sounding characters. The meaning of such characters are usually unrelated and over time on or other of the characters might take on an initial component to distinguish the two characters.
Example:
自 originally meant 'nose'.
It was then used for the character meaning 'self'.
Later 自 was re-clarified, so 'nose' now is represented by the character鼻。