The answer is the Average Total Cost is U-shaped because of the Law of Diminishing Returns.

The law of diminishing returns states that with increasing resources to a fixed process, there are gains in productivity as workers divide the work and save time. Eventually, adding more workers does not produce as much increase in production. The added workers begin to lose efficiency as they try to work in a space that is getting more cramped. The point where the improvements in output grow smaller is the point of diminishing returns. At this point, ATC starts to increase. So adding workers makes the ATC curve go down until the beginning of diminishing returns. Thus the U shape is present in all ATC curves.