Potential reasons:
Hardware fault
Wear of chamber parts, in particular surface roughness.

Chamber maintenance New or refurbished spare parts may result in a change of process conditions, and hence a change of process parameters. Therefore, the stability of plasma parameters is an indicator of successful or faulty equipment maintenance. However, different levels of electron collision rate before and after maintenance do not indicated a fault in every case.

Hard mask

Erosion of a non-resistant hard mask (from deposition such as CVD) can change the process (chemistry) and the plasma parameters. For significant mask erosion or even mask destruction, a large change in plasma parameters and process properties such as CD and yield can be expected.
Resist mask The treatment of the resist mask (annealing), developer and the resist charge play an important role for both the process and its plasma parameters.
Why? Fragments of the resist mask are moved into the plasma and add organic chemistry to the usually inorganic process written in the recipe.
Wet clean During a wet clean cycle, a continuous drift of plasma parameters vs. RF hours can often be observed. This is due to the increasing contamination of the chamber wall (thickness and composition of layer).
After wet clean and reconditioning, the plasma parameters start again at the regular level of the beginning RF cycle.