What is the cause of unusually strong wake turbulence from the 757?
My understanding, primarily based on this archived NASA document (also linked to on the B757 Wikipedia article), is that there were several issues leading to the decision to make the 757 a special case.

First, there were some notable accidents where witnesses seemed to indicate the cause was 757 wake turbulence.

Cessna Citation - December 18, 1992 - Billings, Montana


  Witnesses reported that the airplane suddenly and rapidly rolled left and then contacted the ground while in a near-vertical dive. Recorded ATC radar data showed that, at the point of upset, the Citation was about 2.8 nmi behind a Boeing 757 and on a flight path that was about 300 ft below the flight path of the 757.


IAI Westwind - December 15, 1993 - Santa Ana, California


  witnesses reported that the airplane rolled abruptly and that the onset of the event was sudden. The Westwind was about 2.1 nmi behind a Boeing 757 and on a flight path that was about 400 ft below the flight path of the 757.


Additional accidents were noted with a Cessna 182, MD-88, and a Boeing 737.

Second, there were some controversial NOAA flyby tests which showed:


  for a peculiar set of weather conditions that lasted about 0.5 hour, the vortex velocity of the 757 was approximately 50 percent higher than that of the 767 at similar vortex ages (younger than 60 sec) measured in less favorable weather conditions. However, the results also showed that overall the wake of the 757 decayed faster than that of the 767; in fact, the wake behaved as would be expected for an aircraft of the size and weight of the 757.


Third, they felt the slower approach speed of the 757 (vs. similar aircraft) made it more likely that trailing aircraft would inadvertently close the separation gap during approach.



Ultimately, I believe the correct answer to your question is actually that there is nothing particularly special about the B757 in regards to wake turbulence. This Air&amp;Space Magazine Article indicates that


  Boeing maintains that there is nothing about the wing design that would cause wake turbulence.


and


  "The wake from the 757 is no stronger or weaker than you would predict from the weight, span, and speed of the airplane," agrees NASA's David Hinton, who has taken wake measurements at airports around the country and is principal investigator of NASA's NextGen Airportal Project.


The decision to put the 757 in its own category was controversial at the time and not based on particularly strong evidence. It was done out of precaution due to anecdotal evidence and possibly flawed test results. On the other hand, you could argue that ATC erring on the side of caution is best if there is any doubt. To be fair, a 757 is on the larger side of the small category.