From the Union side the answer would be that Sickles had chosen a really unwise position, essentially in defiance of his orders.
When Longstreet attacked his position en echelon, it was thought too late for Sickles to retreat to Cemetery Ridge in an organized fashion without offering Longstreet an opportunity to breach this weak point in the Union line.
Hence the continual reinforcement of Sickles' Corps all day long and the continual struggle by the Union to push the Confederates far enough back past the Wheatfield and the Peach Orchard to allow the Union to reorganize its line without being breached.
From the Confederate point of view, it was a probing to find a weak point or a way to breach the Union line - given the way the day was going, every Confederate commander involved was justified in believing that one more assault, one more push, might create the opening they needed.
The hope was that Hood would turn the Union flank at the Round Tops and that McLaws might provide a pincers that would enable the Confederates to encircle the Union.
In short, it was the Union trying to plug the hole in its line all day and the Confederates trying to find the hole in the Union line all day.
Many thanks for the reply!