The hireling flees because he is a hireling and does not care about the sheep.
John 10:13
Surrounding context
10:12 But a hireling, he who is not the shepherd, one who does not own the sheep, sees the wolf coming and leaves the sheep and flees; and the wolf catches the sheep and scatters them.
10:13 The hireling flees because he is a hireling and does not care about the sheep.
10:14 I am the good shepherd; and I know My sheep, and am known by My own.