It sounds like you are doing the right thing. This is probably an aerophilic “flor” type yeast that is eating alcohol, and in the presence of air (if it was slightly untopped) decided to have a little fun in the headspace. If you don’t feel the taste or aroma are really damaged or changed, completely topping, plus adding a little bit of extra sulfur dioxide should do it. I would keep an eye on the headspace in the barrel, however. Do you possibly have a leak in the barrel? Is the bunghole forming a tight seal with the bung? Sometimes bungs can harden with time and, due to lack of elasticity, form an imperfect seal.
Did you check the pH? If it’s much over 3.60 your SO2 will have a hard time being effective even if you did add a little bit more. If you suspect you might have a higher pH I would perhaps add 0.5 g/L tartaric acid and stir in gently without oxygenating. Definitely keep the sulfur dioxide at least 15 ppm higher than you normally do, keep that headspace completely topped and keep an eye on it. You may only see this problem once. Then, when you do finally empty the barrel, be sure to clean it extremely well with lots of hot water and a caustic that produces hydrogen peroxide, like “Peroxycarb.”
