Smith had not been acting like himself around the team facilities for the past week or so, sources said.
Bears coach Matt Nagy repeatedly called Smith's situation a "personal issue" without going into greater detail when he was peppered Monday with 12 different questions regarding Smith's surprise deactivation before Chicago's victory over Minnesota.
Smith was downgraded to doubtful 20 minutes before the inactives list came out Sunday. Smith was not on the injury report all last week and the Bears said before and after the game that the decision stemmed from personal reasons.
"Again, he's completely a personal issue," Nagy reiterated on Monday. "For us, it's something that we're gonna leave it at that right now."
Was Smith involved in an illegal incident over the weekend?
"Personal issue," Nagy replied.
When will you decide if Smith travels to London for the Raiders game?
"Again, personal issue," Nagy said.
Will Smith be subject to internal discipline?
"It's completely a personal issue," Nagy said. "I'm not going to answer any more about it. That's where it's at right now. I understand where you all are coming from, but out of respect to this right now, I just want to leave it at that."
The development is an unfortunate twist for Smith, who is considered a cornerstone on Chicago's top-ranked defense.
Smith -- drafted eighth overall in 2018 -- recorded a team-high 122 tackles after appearing in all 16 games (15 starts) for the Bears last season. The second-year linebacker was credited with six tackles in the Bears' win at Washington in Week 3.
The former Georgia Bulldogs standout won the Butkus Award, presented annually to the nation's most outstanding linebacker, during his final year in Athens. Smith was also MVP of the 2017 SEC championship game.