People fall back from the enemy for all sorts of reasons. Sometimes, it is the only tactically sound thing to do, other times it is probably the wrong thing to do. In one on one fights, backing up to maintain your preferred range is just part of the range game. In line fights, it can often set the flow of the fight for those around you as well, possibly even forming gaps or weak points in your line.
So why do lines back up? Oddly enough, it is usually because the other guy is just moving forward without hesitation. Moving with confidence into a range the enemy isn't comfortable with makes them want to back up, just as though they are in a one on one fight. This is even true when a solo fighter presses up against a line, with little chance of fighting at even odds. It is a simple reaction driven into our mindset by countless rounds of sparring.
Of course, if one person backs up, the rest of the line will have a habit of doing the same. Overall, each fighter is working to maintain the line, and rather than leave a staggered gap or be caught alone in a kill pocket, they back up. It isn't the worst thing, at least the line is maintaining its coherency, but isn't always necessary.
There will also be times when falling back in good order is the only tactically sound option. Outnumbered, out armored, and facing a wall of spears, flails and tower shields, your only chance is to fight defensively and hope to buy your team time to win the fight elsewhere. You may also need to wait until the enemy leaves an opening for you to rush.
One other possibility is that it is a trap. Not too commonly, fighters will fall back to lure the enemy into advancing too far forward, into a kill pocket or staggered gap. When done right, with adequate support, it can be extremely effective. The section falling back may also be keeping your attention and positioning your back towards their flankers, which will soon arrive to kill you. If they are backing up with a grin on their face, its probably a trap.
The main point here is to be mindful of why you are falling back. Let the rest of the team know if you need help. Tell the other sections to push if you think yours can hold out on the defensive for a while. If you really can't think of a reason you and the rest of the line are falling back, try to rally them into holding their ground. Just don't use the phrase "hold the line", that word "HOLD" puts a stop to fighting.