The finish of the deadlift should be marked by a solid hip extension, which is created by the hips thrusting forward until the bar physically stops them from travelling any further.
Many lifters, however, continue on by hyper-extending the lumbar spine at the expense of the hips. This is a completely unnecessary and useless addition to the deadlift. Remember that the lumbar spine should be locked into extension from the set up and remain that way until the end of the lift.
The muscles responsible for holding this position do so with an isometric contraction. As such, the shape of the lumbar spine has no need to change during the lift. In some cases, the lifter may simply be unaware of how to properly use his hips during the deadlift, so he resorts to what's more familiar i.e. hyper-extending the low back.
In other cases, the glutes may just be too weak to finish the movement, so lifters rely on using their lower backs excessively to help complete the lift. In either scenario, using less weight and/or going over technique will help to reinforce the correct movement pattern and stop this monstrosity from ever happening again. If not, the added lumbar stress will one day catch up with them.
Take away point: Finish the deadlift with strong and powerful hip extension. Stand tall and resist the urge to lean back.