While my first thought is that it's just dissolved air coming out of solution due to the heat from dissolving the NaOH, another possibility is that the ACRB may be more acidic than usual, if your NaOH is maybe old and contains sodium carbonate at a higher level than freshly manufactured material.
Ideally, you could conclude a little more information about the providence of your materials and what else you may have done in the extraction process (such as acidification) prior to this point. This would help to eliminate some guesswork.
A few bubbles shouldn't be a problem anyhow, although it's a bit weird if you're getting soda-like effervescence with just bark, water and lye. Or did you observe the bubbles just from adding lye to water?