Just like the error states, the witness couldn't reach the principal for 10 seconds.
This could happen for any number of reasons. An extremely high work load that causes the server to be come unavailable, a network disruption lasting longer than 10 seconds, drop network packages, vm issues, DNS issue, etc.
In my past life, we had the witness on a vm, which was over provisioned. So when the server physical server running the VM went into overdrive, it started causing issues. As a temporary fix, we increased the duration of the mirroring timeout from 10 to 30. Once we had a new machine, we moved the VM, which resolved a number of the lost packets and sudden connection issues that we saw.