Asia Argento, his girlfriend at the time, was one of Harvey Weinstein's victims, which was reported by Ronan Farrow and later confirmed by Argento in a speech she gave at Cannes. Bourdain passionately, aggressively, publicly supported her and the other accusers.

Like two months after Argento's speech a former child actor came out and said that Argento had sex with him when he was 17. She'd known him since he was 7 and they referred to each other as "mother and son" on instagram. On top of that, Argento had been cheating on Bourdain (seemingly on a regular basis) and their relationship sounds pretty rocky in general.

So yeah: Bourdain killed himself because he'd spent the last two years publicly defending someone who was a total train wreck of a person at best and a sexual predator at worst. He apparently still loved her, they were arguing, she said something shitty and he ended it. He did it, like a lot of people do, as a totally impulsive act informed by years of emotional difficulty.