Powerlifting competition rules vary from federation to federation, but generally in the deadlift, the lifter is required to hold the bar in the locked out position until signalled by the head judge to lower the bar. At that point, they must lower the bar to the ground in control, which usually just means a fast descent, holding the bar until it is settled on the ground. Accidentally lowering the bar too soon would result in the lift being red-flagged, and not counting, even if a full lockout was achieved.
What you're seeing may be practice for competition, similar to how powerlifters will typically train the bench press with a 1 second pause at the bottom of each rep, rather than touch-and-go. (Bench press rules require the lifter to wait for a command before lowering the bar, leaving it touching the chest and motionless until instructed to press, and then holding it locked out until instructed to re-rack.)