You can do a certain amount of analysis of the general causes of salients. Salients exist when there are two conditions:
- One of the sides wants to occupy or continue to occupy a specific piece of ground despite it being in a more advanced position than the troops around it.
- The other side is unwilling or unable to dislodge them from that position.
In the case of Mule Shoe, Wikipedia writes:
There was only one potential weakness in Lee's line—the exposed salient known as the "Mule Shoe" extending more than a mile (1.6 km) in front of the main trench line. Although Lee's engineers were aware of this problem, they extended the line to incorporate some minor high ground to Anderson's right, knowing that they would be at a disadvantage if the Union occupied it.
That Mule Shoe was high ground is confirmed by the available battlefield maps.
In other words, for Lee to have conceded the high ground at Mule Shoe would have put them at a disadvantage. Lee was therefore willing to expend resources to keep the enemy from doing that, even though the troops defending it would be exposed.
Secondly the Union side, recognising the weakness of the position, made several assaults on it but were unable to dislodge Lee's troops (until May 12th).
This combination resulted in the salient's continued existence.