It could have been that snot-rocket you shot into your fermenter....
But in all seriousness, at what points did you aerate your wort? + at what temperatures was this done. When bottling, did you place your wand at the bottom or just siphon in from the top letting it splash into the bottle? If you give a more detailed description it might be easier to find the culprit. You also could have gotten extremely unlucky and got an infection through something that you couldn't necessarily prevent.