It was quite easy for troops to be surrounded at every point in the war and capture of prisoners happened regularly. Trench lines weren't as close to each other as you might expect and it wasn't always easy to move from the front trench to support trenches. They were usually only connected by communication trenches and if these were blocked or captured then men had to move over open ground in full view of the enemy and artillery/machine guns etc.

Trenches also weren't just straight lines like is sort of implied. They were messy and generally followed contours in the ground. This map demonstrates how complex these systems could be. So depending on which section of trench was captured determined the direction men moved up or down clearing out dugouts and defensive positions. Some trench networks also contained dead ends, other areas for organising troops for counter-attacks and the like. It could be quite easy for troops to quickly capture one section of the line and then by moving along the trenches they could work around strong points of resistance and attack positions from the flanks or rear. When this happened troops generally surrender.

But by late 1917 and into 1918 trench networks almost ceased to exist in that way and instead you get mutually supporting defended points that aren't generally linked by trenches. In cases like that you'd have some troops hold the attention of a defended position, such as a crater or a pill box, and have other troops work around the flanks and rear to attack them there, again forcing a surrender.

There's also times when attacking troops had the element of surprise and were able to get into enemy trenches while its defenders were still in their dugouts making it very easy to capture them. And on the odd occasion a mine could be exploded under a position with understandable results.