Why Did Michael Headbutt Franklin in Option B?

If you choose option b when Devin Weston visits Franklin to demand him to either cut off Michael or Trevor. You will get rid of Michael in a unique mission, during which you also have the option to pull him up over the railing of the tower he gets pushed off and help him up. Choosing this additional option does not save him though because he headbutts Franklin right before he is almost over the railing, but why did he do that? Today we will further explore this question.
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