why lay off and retrenchment always target the low paid workers..why not the other way.. ?

Presumably the higher paid workers who are retained will be better able to accomplish the tasks still required since they are more experienced.

If you laid off the senior workers, you would be left with workers with less experience, and it would be more of a struggle to maintain the level of production.

Also, in the long run, if a company had a reputation of laying off senior employees, no one would want to work for them. Why go to work for a company that has a history of laying off people who have worked for it for a long time? You would always know that even after years of service, if there was a need to reduce employees that you as a higher paid employee would be first to go...