z-Axis Issue - dropping into worktable on first pass

[Woodcrafter (John Matteson)]

If the machine is solid no loose screws/bolts or wiring.

Then another possibility is maybe the cutter got pulled out of the collet during machining.

https://community.carbide3d.com/t/why-did-the-spindle-decide-to-dive-down-into-my-bed-and-rails/67019

Just a thought