Z-Axis raising and dropping substantially during print knocking print off
Can you send a link to a short video of the problem occuring? It might be because of multiple factors but we will be able to identify if there's a video of it.

Possible reasons:


Z axis nut might be too tight. This would cause the nut to bind with the leadscrew and so the motor will not be able to lift the z-axis further while printing.
Not enough retraction. This would cause filament to ooze from the nozzle during travel moves and this filament would collide with the printed object, causing it to be knocked off. A possible solution for this would be to enable Z-hop in the slider, have the hop distance set to at least 1.5x the layer height you are currently printing with.
Too small of a contact area. This usually happens more often for tall prints. A solution would be to increase brim in the slicer settings (at least 10~15 lines) to increase the adhesion of the print and the bed.
Dirty nozzle. Old prints filament may be stuck on the outer face of the nozzle and might be catching onto the printed part while printing.