Jamieson, Fausset and Brown on Job 1:20

20. Job arose--not necessarily from sitting. Inward excitement is implied, and the beginning to do anything. He had heard the other messages calmly, but on hearing of the death of his children, then he arose; or, as Eichorn translates, he started up (2Sa 13:31). The rending of the mantle was the conventional mark of deep grief (Ge 37:34). Orientals wear a tunic or shirt, and loose pantaloons; and over these a flowing mantle (especially great persons and women). Shaving the head was also usual in grief (Jr 41:5; Mi 1:16).