Of all the Zetsus, Black Zetsu should have been the most loyal to Madara. One thing for sure,White Zetsu was quicker to steal and take back the Rinnegan to Madara than Black Zetsu. Black Zetsu was capable of stealing the yang Kurama without Minato noticing, this mean he could have escaped them with ease. Madara had to go for the Rinnegan himself. This means he had doubts, especially its his own will.
Next is when Obito overpowered Black Zetsu. Being Madara's will, Madara should've been able to retrain Obito using Black Zetsu as a vessel. Because of Black Zetsu, Obito was able to steal a portion of the bijuus from Madara.
Black Zetsu thoughts, should be connected to Madara. Anything that black Zetsu know, Madara should know. Not knowing what's going on with his own will, brings in suspicion.
My theory, Madara is suspicious of black zetsu, and step in to make sure that his plan is not ruin. By tapping the ground, he prepped a trap that will help him in the long run.
Next is when Obito overpowered Black Zetsu. Being Madara's will, Madara should've been able to retrain Obito using Black Zetsu as a vessel. Because of Black Zetsu, Obito was able to steal a portion of the bijuus from Madara.
Black Zetsu thoughts, should be connected to Madara. Anything that black Zetsu know, Madara should know. Not knowing what's going on with his own will, brings in suspicion.
My theory, Madara is suspicious of black zetsu, and step in to make sure that his plan is not ruin. By tapping the ground, he prepped a trap that will help him in the long run.