Certain property of a bus admittance matrix
Thanks to The Photon for the heads up. I found the comprehensive answer in following textbook:

Rau, NS 2003, 'Appendix B: Network Equations', in Optimization Principles: Practical Applications to the Operation and Markets of the Electric Power Industry, Wiley, p. 310.

In a nutshell: 

In an admittance matrix the algebraic sum of values in any row is equal to shunt admittance connecting that particular node to the reference node (ground).