Another cause boils down to shoddy workmanship, as it was found that a significant amount of reworking had to be done due to accidental damage.
The report specifically addresses the concern that a new foam formulation developed to meet EPA regulations may have contributed.
"The foam used on the LH2 (Liquid Hydrogen) PAL ramp of external tank #121 was a new type developed to meet new environmental regulations banning the use of chlorofluorocarbons (CFCs) in the application process; however, this foam, known as BX-265, has been used in limited applications on previously flown external tanks. The forward 10 feet of the LH2 PAL ramp was removed and replaced with an improved BX-265 spray process as part of the rework required for Return to Flight. However, foam loss occurred in the aft portion of the LH2 PAL ramp that was not removed and replaced prior to launch.+