Question regarding 2 constructs of tobacco plant
Having a toxic product for the cell in an expression vector is a not uncommon problem when overexpressing proteins. 

The other possibility is that the selection marker itself is the problem. Here you can have mutations either in the promoter of the gene as well as in the gene itself resulting in a protein which is not capable of protecting the cells from the selection. Both possibilities (no expression and mutated protein) leave to the death of the transfected cells, since there is no protection of them by the resistance protein.