They are called Mormon shorts.
This might have to do with what is called "Mormon underwear" . Some members of some mormon denominations wear undergarments that stop just above the knees. here is some more from wikipedia:
LDS Church teachings on use
The LDS Church instructs its members who have been endowed to wear temple garments during the day and during the night.[20] Members are told that they should not partially or completely remove the garment to participate in activities that can "reasonably be done with the garment worn properly beneath the clothing".[20] When necessary, the garment may be temporarily removed, but members are told that after the activity "they should put it back on as soon as possible."[20] Swimming is given as an example of an activity that would justify removal of the garment.[20]
Garment wearers are also instructed that they should not adjust the garment or wear it in a way that would accommodate the wearing of what the church considers to be immodest clothing[20] (which includes uncovering areas of the body that would normally be covered by the garment, such as the shoulders and lower thighs). Members are instructed to keep garments clean and mended and to refrain from displaying it or exposing it to the view of others who may not understand its significance.[20] Prior to the disposal of old garments, members are instructed to cut out the markings on them.[1][21] After the marks are removed, "the fabric is not considered sacred" and the garment fabric may be cut up and discarded or used for other purposes.[1][21]