if the feed was in the bucket for 2 weeks, and you have air born spores in the grow area, if the bucket is not completely covered then the spores can settle in the water, creating bad bacteria in the water, then when you water the plants with this you get pythium overnight, happened to me when i used some old dirt i had laying around, overnight plans were wilted and 2 days later completely dead, wouldnt want to drink a sip even...
another possibility is that the salts binded in the bucket over the 2 weeks they were sitting, but that would surprise me unless it was salts badly mixed or not compatible, or ...a bad batch from say AN(like amoldy batch or something)