Adding a new row in a DGV with no rows with Add() raises SelectionChanged event before you can insert any data (or bind an object in Tag property).
Create a clone row from RowTemplate is safer imho:
//assuming that you created columns (via code or designer) in myDGV
DataGridViewRow row = (DataGridViewRow) myDGV.RowTemplate.Clone();
row.CreateCells(myDGV, "cell1", "cell2", "cell3");
myDGV.Rows.Add(row);