Don't think it would ever be negative, unless when filling out the "199a worksheet by activity" you inadvertently put w2 wages on line 9b  (adjustments) instead of line 9a (w2 wages)

Unless someone likes their job so much that they are paying their employer to work for them, I can't think of any reason to have a negative wage amount other than someone had a case of pilot error while generating the K-1 info.