It's called a couple. It happens with some layouts more than others and with different surfaces. It happens in every scale of airplane.

With a rudder, you can have two different couples. One is a pitch couple, the other a roll couple.

If the rudder movement also changes the pitch, it's called a rudder/elevator couple. And some call it a rudder/pitch couple. When rudder movement causes a roll, it can be called rudder/aileron couple or rudder/roll couple.

If you'd like to experience them both in an example that would be hard to miss, fly a CAP sometime. I've built a couple. Did one for a buddy who's somewhat new at the hobby, and his reaction was, "HOLY S*******!!!!!!!!! WHAT'S WRONG WITH THIS &$^#@&*%$#@(&^%$ !!!!!!!!!!!!!!! " He thought somebody had turned on, on his frequency. Lots of CAPs do both pitch and roll couples quite strongly with even a little rudder.

In your case, you're seeing a rudder/pitch couple.

Back in the good old days, before computer radios, the precision aerobatics guys dealt with that kind of couple by sawing the model apart and glueing it back together a bit differently. They'd move the stab up or down depending on which way the rudder pitched the airplane. And how much they moved it was based on a WAG as to how strongly it was pitched. Today, all you have to do is punch in a rudder/elevator mix into your computer radio. It's easy and contratry to many opinions, it's not cheating or disreputable behavior. It's called "making your airplane fly true" or "trueing the response". And it's fairly easy to do.

The idea about "true response" is simple. You want the rudder to only control yaw. You want the elevator to only change pitch, and the ailerons to only affect roll. And when the airplane does that, it's called many different things, all of the terms being complementary. And there are a number of schools of thought about this deal. Lots of people think you should learn to live with it. They've got a good argument for that until the new model YOU just maidened turns out to be evil. Then it's worth considering doing some TX mixes.