I think heat can just damage the roots like that but soil can also if it is too densely packed....I think it was probably heat issues in your case but I'm just guessing.
Weedhound Reviewed by Weedhound on . Heat Damage~Underdeveloped Roots~Recovery Possible??? in my first grow (log in sig), still going on. im in dirt, indoors. 2 girls seem to have serious bonsai characteristics. long story short.(6 weeks worth) i made some noobie mistakes.(of course) the beginning of the grow i had nearly zero ventilation. used 150w cfls about 1 inch away from first sign of poking out of the dirt. Rating: 5