Bubbles in brownies

Hi to all baking experts: I need some guidance in things that i could do to fix an issue that we are having in our bakery, we baked brownies and recently were having issues with what we think are bubbles in our sheeted brownies, after coming out of the oven they have random bake bubble appearances in the top surface, we have been making them for quite some time and we just barely started to have this issue, could it be over mixing? what can be causing this issue and how can we help solve it?

Any input would be greatly appreciate it. 

Thanks in advance. 

When i watch my favorite bakers on TV  make brownies, they always bang the pan on the counter prior to putting them in the oven to prevent exactly what you've said from happening....over working batter does put too much air into the mix....

Have you switched baking powder suppliers?  Or have you added a little bit of acid to the mix?  A change in acid components plus the baking powder can cause leavening that can cause bubbles you don't want