Fears Of Acting Rude As You Get More Socially Confident
Some people have spent most of their lives being quiet, overly agreeable, and reluctant to express their sense of humor or opinions. As they work on these issues and start to become more confident and outgoing they may have moments where they go too far and accidentally come across as rude or insensitive. For example, they never used to have the courage to kid around and playfully tease their friends. Now they're trying to do that, only to be told their jokes are too mean or crass.
It stings a bit to know they messed up in that particular interaction, and it can also set off deeper fears of, "Is the confident version of me unlikable? Will I lose my filter and act off putting all the time? Is it safer for me to stay meek and shy, and drop this whole pipe dream of feeling better about myself?"
There is some sense in this worry. Some people are too confident and can come across as thoughtless and arrogant. It never hurts to be on guard for that. However, in general it's good to be reasonably self-assured. That's not going to make anyone seem egotistical or annoying.
I find when previously shy, insecure people start acting more confident, but unintentionally come across as a tad jerkish at first, two things can be going on:
A skill issue
They simply haven't had enough practice at the new social skills they're trying to use, and are executing them in a clumsy way. When they try to poke fun at their classmates they have friendly intentions, but they haven't developed a feel for what kinds of jokes are appropriate, and which ones come across as too cutting or vulgar. They just need to put in more time and tweak their material and delivery, not give up entirely on the idea of ever being confident enough to grab the spotlight and bust someone's chops.
Another example would be sharing a political opinion. At the right time and place there's nothing wrong with respectfully telling people your beliefs, or even pushing back a bit against someone you disagree with. Though if you haven't had much experience with friendly debate you may unintentionally come across as too rude, argumentative, or strident. That doesn't mean you should never tell anyone what you think about the world. You just need to get the hang of doing it with a lighter touch.
Old resentments finally having a chance to leak out
Someone who's meek and unassertive can build up a lot of anger and bitterness which they feel unable to express. As they become somewhat more confident and comfortable with humor and sharing their opinions, their anger can start to leak out as passive aggressive jokes and comments. They're still not self-assured enough to directly stand up for themselves in a true confrontation, but at least now they can make little jabs at the people who bother them, which partially satisfies their resentful side.
They may not consciously realize this is what's going on. All they know is that they have this newfound comfort with teasing everyone, but whenever they do it their remarks are a touch too mean and personal, or provocative and annoying. It's like they're trying to hurt particular people, or generally bother everyone and get a rise out of them. Or they feel more relaxed with sharing their opinions, but they keep getting into arguments, which may involve personal attacks or irritating someone by not being willing to drop a subject.
Often this issue resolves once the person figures out what's happening under the hood, and makes an effort to address their resentments directly and communicate in a healthier way. Again, it's not that being confident or outgoing itself is bad, but that in this case the traits were being used as an outlet for bottled up feelings.