At the heart of all is one problem according to advocates: overcrowding. "The SHU is only one piece in the puzzle that is California corrections," says Hadar Aviram, a professor of law at University of California, Hastings College of the Law. "For many years, [the California prison system] was at 200 percent capacity. People were packed like sardines. The conditions were truly atrocious."
In 2010, the Supreme Court responded to the problem by declaring California's prisons unconstitutionally overcrowded. Since then, overcrowding has lessened somewhat, with some inmates shifted from prisons to local jails. The prison system is now at 150 percent of capacity, according to the state.
The American Civil Liberties Union of Northern California sees the overcrowding problem and the high SHU population as not coincidental.
"Corrections officials in California expand the use of solitary because they cannot keep an eye on some of these guys in general population because they are simply charged with overseeing too many guys," Will Matthews, a spokesman for the ACLU of Northern California, wrote in an email to U.S. News.
Callison at the California Department of Corrections and Rehabilitation says there is "no connection whatsoever" between overcrowding and the SHU population, and that the number of people put in solitary units is dependent on how many inmates the state deems a danger to others.
"People are in the SHU for two possible reasons. They commit a crime within the prison system, like a murder, and they can get a determinate sentence," he says. "Or you can also go to the SHU and can get an indeterminate sentence because of a gang 'validation.'"
The state validates an inmate as a member of a gang based on a number of factors. Aviram at UC Hastings says the complicated validation process is part of the hunger strikers' concerns.
"The process to verify is very sketchy, based on things like tattoos or reports from other people," says Aviram. "There are some 1500 [supposed gang] organizations by which you can land in solitary. That is a huge problem from the perspective of the inmates."
The California corrections system says it has made a concerted effort to address these issues in recent years. In October, it launched a pilot program that seeks to provide more clarity to inmates about what will put them in the SHU, and what will get them out.
"The program basically changes the way people are validated about whether or not they are a member of a gang. And it gives them a clearer path to get out of the SHU based on behaviors," says Callison.
More than 200 inmates have been selected for transfer or already transferred to general population as part of the program, according to Callison.
But advocates say the program does little to address inmates concerns about the SHUs, and Mewbourne at Legal Services for Prisoners says she sees "very little difference" between the old and new ways of validating gang members.
That frustration is the basis of this week's hunger strike, which is seeing nearly three times the number of participants as the strikes of 2011 did. And the protests are expected to continue until officials start handing out punishments.
"There are disciplinary procedures for people who violate rules, and people who are leading a mass disturbance," says Callison. "But we're not at that point yet."