TL:DR – There is no need to be really annoyed with people talking behind our back. Most of the time, it is just a matter of convenience. So, cheer up, don’t react… keep your annoyance at bay – too much irritability/annoyance just spoils the day, maybe more.
Why do people talk behind your back? Because, sometimes it is more convenient to do so, rather than having discussions with the subject of the said discussion sitting right there. Why is it more convenient? Because it might lead to a less biased discussion, instead of to a possible argument. People need to talk about their feelings to work through them (or work them out), and it is better done when the subject of those feelings is not present right in front participating in the discussion.
Imagine a situation where you have a serious crush on someone you know, and you need to figure out whether what you have is just infatuation or it could be more than that. Who would you talk to first? That same person, or one of your close friends? Obviously one of your friends, am I right? Say you work it out to “relationship-possible” status and tell the main guy/girl. What if they then simply get annoyed with you for talking about them behind their back, and not expressing your feelings directly to them in the first place? Does that make sense?
Also, just think about what you are doing with this question: You are basically talking about your close ones behind their backs. Yes, you have posted the question anonymously, but that doesn’t change the fact that you did in fact talk about things that they do without letting them know – i.e., talked behind their backs. Now, if you get annoyed when people do that to you, why do you do that to others?
You mentioned that they let you know that such and such discussions about you have happened while you were not present. You should, in fact, be happy that they are so open about it. “Talking behind someone’s back” is actually what happens when people talk about someone in a gossipy and/or negative way, and do not let the person in question know.
What I am trying to do is point out why there is no need to be really annoyed with people talking behind our back. Most of the time, it is just a matter of convenience. At the other times, when it is actually in a negative/gossipy sense, then do whatsaid: “Ignore and move on”. There is no point in wasting any time thinking about such stuff either. If they are close friends, you anyway know there are no bad intentions involved in even the most “negative” things they talk about you – it is mostly all in fun!
So, cheer up, don’t react… keep your annoyance at bay – too much irritability/annoyance just spoils the day, maybe more.