Why is glucose screening test done exactly at 1hr? does blood sugar level only peak then? after that does it drop or can it increase further?

Just a standard way.: Blood glucose testing can be done prior to eating (when you are fasting) and the "1 hour after" is only because it has been done this way for many years and will give a typical indication of your response to eating. It can go much higher in some diabetics and even higher in normal people at an hour after eating. The usual and in my view better indication of your blood sugar status is the HgbA1c