why HI-POT test is done after HT termination..????

It is normal to remove cable from the termination point so that you will only be testing the cable. If you left the cable connected you would apply the test voltage to anything that was attached to the busbar. You would not get a correct reading plus you may damage the other equipment..

You cannot do the hipot test on the cables before installing the terminations, because one of the main purposes of the test is to assure that the termination has been done properly and the cable insulation has not been compromised by that work. If you try to hipot the cable without the termination, the high voltage will probably flash across the end of the insulation at the cut cable end, and fail your test anyway. There is no getting around doing the hipot after terminations are done.

As the other contributors have said, doing the hipot with the cables still connected to the busbars and equipment includes those items in the test, so you cannot isolate a faulty component.

If the cables were originally connected to the busbars, you need to do a better job of supporting them so that when disconnected, they don't slip back and give you difficulty reconnecting. However, that is the way things are in the real world sometimes.