This happens with (old) slow flying planes, like the Bleriot. When you make a turn in a slow plane, the outer wing tends to go a lot faster then the inner wing, where as with fast planes that difference is a lot smaller.
Now when you start a turn in a slow plane with the aileron, making the plane bank, the banking gets magnified as the outer wing goes faster and will produce more lift, lifting it even higher.
If you try to correct this with the aileron, the slow inner wing tries to produce more lift, but it also creates more drag and makes the turn even sharper. So the solution is to use mainly rudder, and a little bit of aileron to assist the turn. Basically always flying horizontal, well almost.