couple things...first, did you get the band tight enough? most banders have an indicator that tells you (some people don't know that and think it's just part of the gizmo that they don't understand). second, we generally try to give a shot of tetanus toxoid (of some sort, whether it be in C&D, or a straight out tetanus shot) 3-7 days prior to banding and another when you band them. third, we cut the sack off 3-10 days after banding. a good way to tell is if the sack itself is cold. use a scalpel or very sharp utinsel soaked in Iodine or Nolvasan or something. fourth, we administer a 1.5cc/cwt dose of penicillin at the time of scrotal removal, just in case there is an infection that you may not even detect yet.
my thoughts, your band wasn't tight enough...maybe from something as minimal as an abnormal testicular development. since you probably a) didn't cut the sack off soon enough and b) didn't give penicillin when you cut the sack off, then there was nowhere for the infection to go but up. sorry, I learned the hard way too.