Military history isn't my particular specialization, but this was a subject we talked about in some depth in my US military history class in college.

What we learned then was that the shift primarily had to do with the advent of Napoleonic warfare: instead of relying as much on lining up and fighting via trading volleys, Napoleon's tactics relied on the ability to move quickly, attack, and finish off remnants with cannon and other similar fire.

By the time of the US Civil War, however, these tactics are inefficient: the first rapid-fire guns are being used, minie balls have been invented, and strategies like massed charges no longer result in clearing out great swaths of the enemy; now, people just run headfirst at a weapon that will mow them down. As this was explained to us, while technology had advanced, tactics remained in a rudimentary Napoleonic phase that had not yet adapted to the new technology. Trench warfare at this point wasn't widespread, but I remember reading accounts about how soldiers on either side, when they got in skirmishes in the woods, would dig holes and try to wait out the conflict there, an early example of trench warfare.

The use of trench warfare in WWI can be viewed as an attempt to adapt to the changing conditions of warfare, and I would say that between the first and second World Wars, the advent of new military technologies (tanks, the first planes/air forces, etc., dreadnoughts) hints that modified tactics in WWII were attempts to adapt to/incorporate these new technologies.

Sources:

For the Common Defense, Allan Reed Millett