Why was General Thomas J Jackson anointed with hero status after the Battle of Bull Run?

Because he held his position with great difficulty, as noted by Brigadier-General Barnard Bee, who shouted "Look, there is Jackson standing like a stone wall!"

He is supposed to have added "Rally behind the Virginians!", but the other version claims he was criticizing Jackson for not coming across to help his own brigade.

As Bee was mortally wounded the same afternoon, nobody was able to establish what the true message was. However, the 'Stonewall' nickname became established, and Jackson's tactical skill and almost mad gallantry were never in doubt until his accidental shooting at Chancellorsville two years later.