I am a very active, fit, nutrition/health conscious 41 y/o. But I'm "pre-diabetic" despite no family history. What could be causing this?

Pre diabetic. Means that you are on the way to developing type 2 diabetes. Remove ALL sugar and sweeteners from your diet.
Genetics. It may simply not have expressed itself in past generations -- there are diabetes genes that are recessive and powerful. You may also want to get checked for hemochromatosis. Keep exercising and you may well stay fine.