Spoilers ahead for the October 14 episode of 9-1-1 on Fox, called "Triggers."
9-1-1 got off to an intense start in Season 3 thanks to the massive tsunami that hit the Santa Monica Pier and created a truly titanic catastrophe that even Ronda Rousey couldn't punch into submission. Buck, who had been benched from the LAFD after his health crisis at the end of Season 2, had been at the pier with young Christopher when the tsunami hit, and they went through a terrifying ordeal that they were lucky to both survive. Now, however, it seems Buck may have survived the tsunami only to lose everything in the aftermath.
Buck was actually surprisingly upbeat at the beginning of the episode considering he was on "light duty," a.k.a. working as a fire marshal. It turned out that Buck was under the impression that it was only a brief matter of time before he would be cleared to return to duty as a firefighter, despite the fact that he was on blood thinners. He was upbeat, although somewhat discouraged to see that Lena Bosko had taken his place under Bobby's command at Station 118.
It was clear that Bobby had something he needed to tell Buck, but he didn't especially want to do it. His hand was forced when Athena, believing that Bobby had broken the news to Buck already, invited Buck to dinner. It was a cheerful dinner on Buck's side until Bobby finally had to drop the hard truth: it wasn't the LAFD brass who weren't letting Buck return to duty as a firefighter. It was Bobby himself, who believed that Buck just isn't ready.
Unsurprisingly, Buck exploded at this reveal and stormed out of the house. In fact, he was angry enough that he went to a lawyer he'd encountered during his work as a fire marshal. The lawyer had originally reached out to Buck, believing that he was a "disgruntled" employee due to being benched. Once Buck was genuinely disgruntled, he came to a shocking decision: he's suing for his job back.
Yes, Buck made the potentially catastrophic decision to pursue a legal course of action. Although he was clear to the lawyer that he only wanted his job back, therefore wasn't looking for damages or a payout, the lawyer made it clear to Buck that there could be some serious consequences if they lose the case. If the lawsuit doesn't go Buck's way, then it was all but impossible he'd ever work for the LAFD again and he likely wouldn't be hired by any other city, having gained a unfortunate reputation.
Then, the lawyer made it clear that if they pursue the suit, Buck shouldn't maintain contact with any of the people who were involved, presumably even tangentially like the rest of Station 118. After considering this, Buck stated that he didn't really have anything to say to any of them anyway.
The episode revealed Buck's ultimate decision at the very end, when Buck dropped by Bobby and Athena's house to apologize for rushing out of dinner. Then, he dropped the bombshell: he's suing the city of Los Angeles, the fire department, and Bobby himself for wrongful termination. Say it ain't so, Buck!
"Triggers" set up Buck's future in a way that suggests he could lose everything if he loses the lawsuit, and he's likely burned some important bridges by pursuing legal action in the first place. If he wins, he could get his job back, but lose the trust and friendship of some who were very close to him. If he loses, he loses big.
At this point, I'm really wondering if this means Buck will stay away from Eddie and Christopher as well. Buck is very close to Eddie, formed a bond with Christopher, and Christopher loves him. Angry as he is, would Buck really punish Christopher by staying away, when Christopher is already struggling in his own way following the tsunami?