What causes igg deficency ?

Many causes: If your IgM and IgA levels are normal or high, it's likely you have one of the still-not-fully-understood genetic glitches that come under the heading of "common variable immunodeficiency" which is very common. If your IgG is just a wee bit below the published "normal", consider ignoring it, but if one or more classes is way low, you deserve an official diagnosis.