Adm Mullen “I think the firing was a really bad decision,because it undermines the authority of the military commanders who are trying to take care of their troops and significantly negatively impacts the willingness of commanders to speak truth to power” https://www.washingtonpost.com/opinions/2020/04/04/trump-wants-him-fired-inside-ouster-capt-brett-crozier/

According to this behind the scenes article, it was civilian acting Sec of Navy, after receiving input from Trump, who overruled Crozier’s military command chain, who wanted an investigation of the circumstances leading to Crozier’s letter instead of this unusually quick firing