Z moving on it’s own is definitely a well reported problem like here.
As suggested there, one should always check the cable connections.
But it is still useful to rule out the g-code. Once you have settled down - from being pissed off that is. Set the Z 2 or 3 inches up in the air with no work on the table and run the same code again. If the problem is EMI or connectors or something else it will not happen in the same spot each time.