The man runs away because he is a hired servant and is unconcerned for the sheep.
Berean Standard Bible (BSB)
The hired hand flees because he is a hired hand, and doesn’t care for the sheep.
World English Bible (WEB)
{G1161} The hireling{G3411} fleeth{G5343}{(G5719)}, because{G3754} he is{G2076}{(G5748)} an hireling{G3411}, and{G2532}{G846} careth{G3199}{(G5719)} not{G3756} for{G4012} the sheep{G4263}.
Strong's King James
(13) Because he's a hired hand and He's not concerned about the sheep.
The F.O.G
The F.O.G Message
and the hireling doth flee because he is an hireling, and is not caring for the sheep.
Young's Literal Translation