Seems like they should at least finish the investigations before firing people.
Not necessarily. If there is strong enough evidence (even if it isn’t official or conclusive) it is considered good practice to take them out of the equation sooner rather than later, usually by relieving them from command and assigning them somewhere else pending an investigation.
I think that is a wise thing to do (not that there is a huge amount of wisdom going on here) because think about it: If initial indications point to a failure of command/leadership/capability, do you really want to leave a combat unit or ship under that same commander for months while the investigation takes place? I don’t think I would.
If there is no fault there, they will resume their career. Like another poster stated, I don’t like seeing officers getting relieved, but that is secondary. The primary issues (in this order) must be mission readiness, safety, and leadership.