Reason doctors not letting baby stay in womb for not more than 2 weeks pass due date could also be - it may cause serious risk to mother's life also?

Yes that is: Possible. It can put mother at higher risk for infection and bleeding after delivery. And possibly needing a csection.

Some basics: The umbilical cord & placenta are a time limited system. As it approaches the end of pregnancy it begins to wear out & the nutrient transfer membranes begin to deteriorate. Statistically the chances of a stillbirth/infection/ etc. as you pass that point begin to increase dramatically. Get baby out before this happens and both are safer.