Can cabin pressure be lower than outside pressure?
Besides the sealing the aircraft would also need a vacuum pump that is able to pull air out of the cabin which means extra weight.

The way aircraft maintain cabin pressure is through a air cycle machine that pushes air into the aircraft and outflow valves. There is no facility to pull air out of the craft. 

Any pressure drop you may experience on the ground is probably because the pilot increases air pressure on the ground slightly to test the seals and then releases it before takeoff.