Either one: you're going to have to answer that one. You might be doing something wrong, you might be getting more frustrated if your practice is not balanced properly. If some people focus too much on simply getting concentrated to obtain a calm peaceful mind state, they may get angry at any outside interruption. Then they are not practicing correctly and they might be getting more angry. So if you don't understand how to use mindfulness to note these contacts and the problems that arise from them, then it might be true that you don't understand the practice correctly.
However, it's also true that many people all of a sudden see the anger that was already there. Because that's what this practice is for, we're going to start to look at everything. A lot of people never know how much anger they have. A lot of people never even know it's Dukkha to be angry. Here we're going to start looking at our conditioning and the conditioning that creates anger, and all of a sudden we might think, "Oh my gosh this is more than I ever had before," but often it's just simply what was always there that we never saw before.
Keep in mind the little example I give about the practice being similar to cleaning a big house. If you don't look and find the anger, you can't clean it. It's often hiding in the cupboards, it's often hiding in a bathroom, wherever. So we actually have to go looking to discover what is actually already there. And frequently indeed, in the beginning a lot of people are very amazed at how much anger, in particular, they actually have inside themselves. But that's good because now we have the chance to work with it to let it go.