Williams explained the decision-making process during an appearance on SiriusXM NFL Radio (via CBS Sports' Cody Benjamin).
"It was hard," he said. "My mom is my rock, my everything. ... My mom, she just was like, 'Whatever decision (you make), I'm behind you.' ... Football is my life, especially coming off a championship and wanting to do a repeat—which, you know, I know they're gonna handle that. But at the end of the day, this was something personal."
Chiefs general manager Brett Veach announced the 28-year-old's decision Wednesday.
"As an organization, we certainly understand and respect Damien's choice, knowing it was made in the best interest of his family," Veach said. "He means a lot to our football team as a player and a person, and we're going to miss having him around this season."
Williams led Kansas City's deep backfield with 498 rushing yards during the 2019 regular season. His most important performance came in Super Bowl LIV as he recorded 133 yards from scrimmage and two touchdowns, including the go-ahead score, in a 31-20 win over the San Francisco 49ers.
The undrafted free agent out of Oklahoma was slated to lead the Chiefs' rushing attack again in 2020. Instead the team will probably lean even more heavily on a committee approach.
Rookie Clyde Edwards-Helaire, the team's first-round pick in the 2020 draft, will likely finish the campaign with the most touches. Darwin Thompson, Darrel Williams and Elijah McGuire are among the other backs who should be involved during the course of the year.
Of course, the running game plays second fiddle to the Patrick Mahomes-led passing attack in K.C.
Since Williams' decision to opt out isn't based on a personal high-risk designation related to COVID-19, he'll receive a $150,000 advance on his contract as part of an agreement between the NFL and the Players Association, per Benjamin.
The San Diego native will remain under contract with the Chiefs for 2021 as part of the two-year, $5.1 million deal he signed with the organization in March 2018.
Kansas City, in its role as the reigning Super Bowl champion, will kick off the new season Sept. 10 against the Houston Texans at Arrowhead Stadium.