First, it takes a minimum of 2 weeks for a dog to come out of their shell and show their true personality.
Second, for a dog to have another animal by their neck and shaking them it is serious. That is how they kill.
Since you were not out there to witness what happened it's hard for anyone to say. However, going forward, do not leave them unattended for a second. Crate and rotate if necessary. It takes a couple weeks for the stress hormone to leave their body after a fight so until then, it's not uncommon for the fighting to escalate.
Jax and Sierra used to fight. Sierra has terrible nerves and is dog aggressive. She never once came out on the winning end and was at the vet for stitches more than once. It only take a glance to set a fight off. The trigger can be very subtle. We started training with them to get it under control but then I went on vacation and took Jax with me. We haven't had a single fight since then. It's like something reset in Sierra's brain.