When using a filament based printer, what operator behaviors increase the frequency at which a bed must be relevelled between prints?
Operator induced regular actions
When an operator reaches into the machine and operates something on the bed, this can induce errors that slowly accumulate. The most typical operation would be to remove something from the printbed, cleaning the printbed or swapping the printbed wholesale all can result in a slow but steady unleveling.
Regular leveling can counteract this. With good training, you might vet away with once in a dozen or less.
Maintenance of/work on the printhead
During maintenance such as swapping nozzles, checking connections and cleaning the printhead, there is a very high chance that the printbed is touched due to the usually very cramped area one has to work in. For example, it is near impossible not to touch the printed when swapping nozzles on my Ender3, if I want to use my torque screwdriver. This is true even with the Z-Axis at the highest position, due to the dimensions of my torque screwdriver.
By virtue of the work on the printhead, the 0-level is usually thrown off anyway, and as such a relevel is always in order after any printhead maintenance more invasive than cleaning the fan ducts.
Avoiding nozzle swaps unless necessary can reduce the workload - it might be cheaper in the long term to have two machines with different setups than one machine where you swap the nozzle for each print - unless you charge for the accompanying work on the setup change.
Operator induced irregular actions
There are cases where the operator did not plan to operate in the area of the printbed but actually might impact it by reckless or accidental action. In other words: accidents happen, tools drop onto the printbed and hands end there if an operator stumbles.
While releveling might not be necessary after all such accidental contacts, occasionally checking it and fixing it is good practice.
The way you test and maintain your Z-level is paramount in how often you need to validate the Z-level. If your springs are too strong and push the leveling knobs off on their own or your Z-sensor is mounted only weakly, then these create problems on their own.
Note that even in normal operation, the oscillation of the printer will make any bolt under stress that is not glued in place or jammed in tight loosen a little over time. This does include the bed leveling knobs.
The only ones I have found are.
Manually putting pressure on the bed when removing prints.
Removing the bed covering, eg a glass plate
Damaging the bed in some way. For example my bed has high spots on it (always has). This means that if I remove the glass plate I use and put it back, it sits slightly different. If I orient the glass a different way from prior it always needs levelling.
Changing filament types.
Changing cover types eg magnetic and glass are different thicknesses.
Changing first layer needs. Sometimes I need the first layer squished a bit depending what I'm doing.
Lastly on my Ender 3 Pro if the z-axis switch isn't screwed in tight enough it can slip down a fraction.