First we need to understand that not a lot of time has passed between madara getting hit by hirudora and the juubi finally being revived. This can be seen in the picture of madara because he still has smoke and dust flying behind him, away from the juubi so it's not the juubi that caused the smoke around madara. Next, we need to understand that gai got a surprise attack on madara while he was looking away. Hirudora most likely barely did any damage but the pure force of the attack was enough to looden his grip with the move. Hirudora is a formidable attack so its understandable that his grip would loosen, since he was sent flying.

Still madara was a straight up boss in the one and only panel he had.