Sometimes I notice that if I manually command a single axis movement (typically Z, when I want better access to the extruder), I observe that several channels move together (and they maybe move slower than I expect). After one 'coupled' movement, subsequent commands have the result I'm expecting.
What happens is as well as the Z-axis moving up, the bed moves forward, and the extruder moves to the right. I have no auto-leveling or anything else non-standard on this printer (dual Z steppers, X, Y, extruder, bed, extruder heat).
It even happens if I simply extrude some of the time (e.g. changing filament after warming up, retract gave me some X movement)
Printer is an ANET-A8, I'm using mainly OctoPrint, but I think I've also observed this with other PC software manual controls.
It doesn't happen often enough for me to have identified any pattern - maybe it only happens if I've not homed first after turning the printer on, but I suspect not.
It's not so much of a problem, as just a question for interest. Also not sure how to tag.