From an excellent story in The New York Times: 

Retired N.F.L. players have flocked to California in recent years as word has spread about its workers’ compensation system. The state is believed to attract more football-related claims than all other states combined because of two quirks that suit them perfectly.

Most states require workers’ compensation claims to be filed within one to five years of the injury; California’s statute of limitations does not begin until the employer formally advises the injured worker of his or her right to workers’ compensation. N.F.L. teams have almost never brought up workers’ compensation — hoping to avoid even more claims, several lawyers said — so long-retired players can file for injuries sustained decades ago. Dozens of veterans from as far back as the 1960s and ’70s, including the star San Diego Chargers wide receiver Lance Alworth, who retired in 1972 and turns 70 in August, have California cases pending.

California’s other crucial wrinkle requires a professional athlete to have played only one game of his or her career within state borders to file a full claim for cumulative injuries. The law derives from California’s desire to protect outside workers who temporarily pass through the state, like truckers or flight attendants.

Leroy Thompson is an example of how the concept operates to athletes’ advantage. A reserve running back for four non-California teams from 1991 to 1996, Thompson qualified for California workers’ compensation because 4 of his 80 regular-season games were played there. In January, he accepted a $120,000 lump sum to settle his claim. His original 2008 filing asserted cumulative injuries to his “head, neck, back, spine, shoulder, hips, elbows, wrists, hands, legs, knees, ankles, feet” and other body parts.