It can be a passive-aggressive way to call you out, but not necessarily. If you and your SO have a healthy relationship, discussing problems in front of others shouldn’t necessarily be seen as an attack. In my experience, if a situation like this comes up, it’s usually either:
A way to help a friend who that might not realize they’re in a toxic/abusive relationship. For example, “Sometimes, Niels is really impatient with me being late, but he doesn’t demean me or put me down” can be a way of inviting me to provide some context or show that I’m not perfect but also that there are hard lines that shouldn’t be crossed
A way to empathize with a friend that is coming to grips with the fact that no relationship is perfect. It might not be a conscious invitation to participate, but it does potentially allow you to provide another way of thinking about the issue. For example, “Tell me about it; I’m always picking up after this guy” is not mean-spirited or passive aggressive, IMHO. If it seemed like they actually wanted me to participate rather than just have a way to vent, then depending on the friend I might jump in with “oh man, my ADHD has been really bad since the new project started at work! I’ve been a mess and it’s been really awesome that you’ve been able to help me out”
It’s still not an excuse for an SO oversharing something that you told them in confidence, and it’s not an excuse for turning an ongoing point of contention into a full-fledged argument in front of others, but it’s not necessarily cause for concern. If you’re a very private person, and you ask an SO to not discuss certain things with others, that’s a little different. But on the other hand, they do need to be able to discuss concerns with others to some extent, so maybe differentiating between “good friends are OK, but not that friend” would be helpful.