Cable too loose in the housing. Needs to be quite tight, making sure all ferrules are properly seated along the entire length. This can be a little tricky--if you don't have a "third hand tool" is easy enough with pliers or vice grips, hex-head wrench, etc. placed where you can reach them while holding the cable. Your problem probably came about by one of the ferrules slipping out of its housing or a section of cable housing slipping out of the ferrule while you were juggling the tools trying to tighten the brake. Seemed tight enough at first, then when the cable components slipped back into proper alignment, you got a case of (blush) saggy cable.
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