There are several possibilities.
1. Too much power too early in the rod stroke, which causes the tip to dip down initially. Correction - start smoothly.
2. Rod creep on the forward casting stroke compensated by too much power late in the rod stroke to make up for loss of stroke length due to the rod creep. Correction - drift the rod back after the back cast stop to gain extra stroke length.
3. Aiming too high at the stop to get extra length before the line falls to the water. This causes a rod tip path of less than 180 degrees. Correction. Make sure your forward stop is not less than 180 degrees from the backcast.
All are faults that occur when trying for that extra distance. Ever wonder why you don't get a tailing loop while false casting but do on the delivery cast? It can be because you are trying to cast too far on the final forward cast and you shock the fly rod with a sudden acceleration. This bends the rod causing the rod tip to dip below the SLP resulting in a concave rod tip path.
To get that extra distance double haul and learn to shoot line on the back cast that is before the delivery cast so that you don't need to shoot as much on the delivery cast.