When you keep attacking a Banjo Banana when it's in the back, it might fight back. A yellow exclamation mark will appear above it to warn you, as shown in the image below.

Look! Banjo Banana just started his karaoke attack on you.

Tap Banjo Banana with your finger to make him stop singing.

Note: Banjo Banana has a chance to retaliate with an acorn ONLY when it's in the back.