Low level pylon racing in aviation is roughly equivalent to being a circus high-wire artist without a net. There is very little margin for error, you are operating 150 or so feet above the desert floor, and as the old pilot saying goes, one of the most useless things is altitude above you.
The accident that claimed the life of Jimmy Leeward and at least 10 people on the ground is not the first time that a pilot in the Unlimited class has perished in a race heat at Reno. There have been other fatalities over the years. Racing converted and re-engineered World War II machinery at low altitude and high speed is a high-risk endeavour.
There has been much speculation on the cause of the accident. However, out of the usual cloud of superficial media reporting, and speculation, some facts have begun to emerge. An onboard video camera was running in the cockpit of Galloping Ghost during the race, and other video not so far appearing on YouTube has been examined. Here is some informed analysis of the events from a private message board that I am a member of:

...the cause of the crash was due to The Galloping Ghost having a CG too close to the aft limit which resulted in pitch instability. There are instructions on the P-51 regarding no combat missions with the aft fuel tank full resulting in an aft CG problem. Instructions specify to empty the aft fuel tank first in flight.
During qualifying my friend watched Galloping Ghost from inside the cockpit of a following plane and could not believe how much trouble Leeward was having in keeping the Ghost in a stable pattern around the course.
Since Leeward lived in Florida and the Galloping Ghost was modified for racing in California, when Leeward picked up the Ghost for the Reno races at the last minute, a complete flight test program had not been done based on available information.
There is a video of the entire last lap of the Ghost before the crash which my friend showed me. As Leeward was coming around pylon #8 at about 480 mph after passing Rare Bear, he hit turbulence which pitched his left wing down, Leeward corrected with hard right rudder and aileron. Just as the aircraft was straightening out, he hit a second mountain of turbulence which caused the tail to 'dig in' resulting in a 10+ G climb rendering Leeward unconscious instantly and resulted in the tail wheel falling out. (broken tail wheel support structure was found on the course). As the Ghost shot upward the LH aileron trim tab broke loose. This can be heard on the tape, so the trim tab did not cause the accident.
Since the Ghost was racing at 480 mph with full right rudder and the stick full right, this is where everything stayed when Leeward blacked out. Cockpit camera film that was salvaged from the wreck shows Leeward slumped over to the right in the cockpit. As a result, the Ghost climbed up and to the right, rolled over on her back and then headed for the box seats. Most in the box seats never saw it coming because it came in from behind them.

UPDATE - The NTSB has now issued the full final report on the accident.Section 2.1 confirms the tentative conclusion above that events occurring as the plane rounded pylon #8 caused g-loadings on the pilot to exceed levels that any human could tolerate. From the report:

...during the roll events, the airplane also rapidly pitched up. The characteristics of the pitch changes were such that, within about 1 second, the vertical G levels rose from about 3 G to 17.3 G; both the rate of G onset and the high-G level exceeded human tolerance. Based on available guidance, the accident flight’s rapid rate of G onset and the high-G levels were such that the pilot’s time of useful consciousness was likely less than 1 second. During the brief interval before losing consciousness, the pilot’s physical performance would have been limited due to the increased load on his body and extremities and the decreased blood flow to his brain. As a result, the pilot soon became completely incapacitated (which is consistent with photographs showing that the pilot was slumped in the cockpit during the climb), and the airplane’s continued climb and helical descent occurred without his control.

However, the underlying root causes of the accident, which are discussed in the report, may lie in a number of modifications made to the plane to increase it's low-level racing performance, which were subjected to insufficient analysis prior to execution. From Section 1.3.1 of the report:

The reasons for many of the modifications could not be established. Neither the pilot’s family members nor the airplane’s ground crew were aware of any detailed drawings, engineering calculations, or other substantiating data for any of the modifications.

Although the immediate cause of the high-g manouver that incapacitated the pilot was a failure of the elevator pitch trim tab assembly, photos in the report show several instances of deformation of other parts of the airframe structure both during the accident flight and earlier flights at the same event. The photos show an airframe that was being highly stressed by a race duty cycle far in excess of anything that the airframe had ever been subjected to in any prior racing activity. Section 1.10.1.2:

A comparison of information from the accident race, previous races, and qualification flights (all flown by the accident pilot) showed that the airplane’s maximum 458-knot GPS ground speed between pylons 6 and 7 during the accident flight was the fastest that the airplane had flown on the course by about 35 knots. The accident flight had the highest engine manifold pressure and rpm (compared with all previous flights for which there were data) by about 15 to 20 inches of mercury and about 100 to 150 rpm, respectively.