why does H3SiNCO show back bonding even though it has enough e- to satisfy all valencies.

A structure can become more stable with more delocalization  of lone pairs / or charges .Here lone pair on N can undergo delocalization in the empty d orbitals of Si which results p pi and d Pi back bonding .
Based on it we  have more different questions like
trimethylamine is trigonal pyramidal however trisilylamine is planar?

Maam understood what u said but won't trisilyamine be 3d instead of planar.(DUE TO ALL THOSE SINGLE BONDS) ??

ipl check the picture ...Can you see double bond character ?

Let us take Si- . It has 4 groups around it. How can all 4 of them be planar.