You don’t have to be an expert. “Now He who searches the hearts knows what the mind of the Spirit is, because He makes intercession for the saints according to the will of God.” - Romans 8:27 (NKJV)
They were envious.
“These last men have worked only one hour, and you made them equal to us who have borne the burden and the heat of the day.”
- Matthew 20:12 (NKJV)