I am making a 2-layer 9" round. The BC keeps pulling away from the sides of the cake. What causes this? There will be a bubble sticking out on the side like there is too much air underneath.

Sounds like your buttercream is too thick.

OK, thank you so much.

Is your cake wet? This can cause the buttercream to fall off too and bubble. Sometimes if you freeze the cake and it doesn't fully reach room temp, this can happen. Just thin out your icing and this will help