The fuel of rebirth are volitional karmic formations that we usually identify as plans, intentions and tendencies. They can be bright, dark and mixed of nature (SN12.38)
It is not that Buddha teaches that asmi mana, the notion I am, the notion of a self or ego, is the cause of suffering and rebirth. The second noble truth of the cause of suffering is tanha, not asmi mana.
But the notion “I am” itself arises because of clinging (SN22.83). Not without. Mind is never always with clinging and therefor also not always with an ego notion. Also not the mind of a worldling.
But once arisen due to instinctive clinging , i believe, the ego notion, works like a katalyst. It only feeds and speeds up and intensefies the allready subconscious started clinging, attachment, distortion.
Even when unconscious there are still instincts that protect body and mind. This protectiveness is suited very deep.