Is there a Risk of Tail Tipping during Push-back of a passenger aircraft?
During pushback the (usually quite substantial) weight of the tug pulls the nose down, and additionally when it's pushing backward the line of force is below the CG and acting rearward, so that would also tend to counteract any tipover tendency.

As Jan mentions in comments, the tipover situations tend to happen during loading when the CG is moving around. I suppose if the tug were first attached and then the aircraft were grossly misloaded, it's conceivable that it could tip over backward as the tug detaches and removes it's weight, but I've never heard of that happening.