The film 1917 takes place in early April of the title year. This was a watershed moment in the war in regards to German strategy on the Western Front. Up until this point, the German army was committed to defending and holding every scrap of ground that it had captured, emphasising immediate counterattack in the event that the British or French captured a section of their line. This resulted in a rigid, inflexible defensive position with many German troops occupying frontline positions in range of British and French artillery.

Nine months before the movie is set, the British launched the Somme Offensive, the great operation that was intended to smash the German frontlines, break through and resume a war of movement. That breakthrough never materialised and the British (and some French) spent the next several months doggedly assaulting one German position after another. This resulted in high casualties for both sides and advances in British and French artillery practices led the Germans to re-examine their defensive strategy. Because German doctrine emphasised counterattack and the recapture of lost ground, large numbers of German troops were repeatedly funnelled into areas that were subject to heavy artillery bombardment. Both sides suffered from this but it was particularly concerning for the Germans who were fighting a two front war and were losing their most experienced units.

The re-evaluation of their defensive strategy led the German army to adopt a defence-in-depth doctrine. Instead of a thin, densely occupied frontline, the German pivoted to a thinly held frontline and successive lines stretching kilometres in places. Essentially, this placed the bulk of German forces beyond the range of British and French artillery and any attacking force would have to advance through a deep, sparsely occupied zone encountering increasing levels of German resistance. By the time the attacking force came into contact with the main German force, it would be depleted, tired and without the artillery necessary to overcome German defensive positions.

This defensive posture would continue until the latter half of 1918 when Entente forces finally achieved an operational capability to overcome such extensive defences and resume a more mobile style of warfare. The reason this is important to explain is because it ties directly into the events of the film. By April of 1917, the Germans had completed construction of the Siegfried Line, known to the Entente as the Hindenburg Line. This defensive line was built in secret behind the German front line (in an area that has not seen heavy fighting hence the grassland) and was built to allow the Germans to pivot to their defence-in-depth strategy. In order to occupy this new defensive line, the Germans needed to withdraw from the existing positions. They did this throughout the months preceding the events of the film and managed to do it without alerting the Entente to the actions. Before withdrawing however, the Germans left booby traps for the British and French forces that would inevitable advance in the wake of the German withdrawal. This is seen in the film when the two British soldiers inadvertently trigger an explosion in the abandoned German dugout. They also buried shells beneath roads and in the cellars of buildings where they knew soldiers liked to seek shelter. They cut down orchards and poisoned wells, drove of or killed livestock and destroyed bridges, all of which is seen in some form or another in the film.

When the Entente did become aware of the German withdrawal, they did indeed cautiously advance in early March, eventually encountering German resistance again in early April, when the film in set. While I haven't researched the events depicted at the end of the film intensely to verify that the depicted attack did indeed take place, I can say that it wouldn't be surprising that the trenches depicted were freshly dug (though those seen in the film were quite clearly dug by machinery rather then by hand) as the first instinct for any force by that time of the war would be to seek shelter and prepare positions from which to launch an attack. The forest seen would not have sufficed as cover as exploding shells created deadly splinters that did more damage to sheltering soldiers so trenches were the preferred option. The trap itself is not something I am aware of happening but rather (I believe) to be the filmmakers way of explaing that the Entente for the most part didn't know how significant and extensive the new German positions were and that they had been designed with specific killing grounds in mind to funnel attacking forces towards waiting machine guns.

To end and summarise, the film 1917 does depict a very small window in which more mobile operations resumed on the Western Front but a window that was created by a German withdrawal to a more defensible position.

Source:

The First World War by John Keegan

The Somme by Prior and Wilson

For certain parts, it was indeed a mobile war. The appropriate section of the FAQ is most helpful here, most especially the ones from the sadly now-deleted u/elos_; in particular this overview of the war and this post covering how combat changed over the war. The second one is particularly instructive as regards your question.