This is my wife in a nutshell.
I love her, but due to hormonal imbalances, she can be very very (exhaustingly) difficult (stress/anxiety...). Day to day, she's ok, and with her friends, she puts on a facade, but if there's the slightest bit of stress or anxiety, I'm a punching bag.
A great example are holidays, and I've said this to her... She loves them, I love them, but the JOURNEY is a nightmare. I try my best to literally avoid her until we are actually at the destination, as the travel to the airport, checking in, passport control, the stress of the flight, and all the travel at the destination before we get to the hotel makes her insanely anxious, and even asking whether she's hungry will result in being snapped at.
I've posted on MN before about how we used to go on literally a city break a month, and maybe the odd long-haul trip too, but we've cut right down. She asks me to book holidays, or make suggestions, and she has occasionally even resorted to 'spite holidays' with friends without me because I've just lost all motivation to go. I'm not going to stop her going alone, a bit of an escape is actually nice... of course I'd never say this to her.
Dinner is another one. I do the shopping and cooking. I ask around midday whether she'll be ready at 5 for dinner. I then ask again around say 4 to check, and start preparing. Things like stir-fry, you can't really reheat or 'pause'. It's ready at 5, so I'll say it's ready and I'll just get snapped at "Yes, I'm working!". If I then eat on my own, it's passive aggressive, but if I wait to start cooking until she's done, or don't remind her that it's 5, it could get to 7 pm and then I'm snapped at because it's going to be another half hour until it's done or that it's too late to eat...
It's any stress, any anxiety. People will hate me for saying this, but it's like I live by a keeper's manual... don't ask her anything before midday, don't ask what she wants for dinner, if we travel, make sure there are snacks in the car, if there's traffic-keep a low profile. My brain has a filter on it now, almost rehearsing 20 ways to say something in my head from experience of all the times an innocent comment has been attacked.