How would I explain it?

What would explain the change in reaction rate —degradation of cellulose by the enzyme cellulase—after 75 minutes is the depletion of the substrate on which the enzyme must act.

Explanation:

A chemical enzymatic reaction, such as cellulase degradation of cellulose, occurs until the substrate is consumed.

The solution to which cellulase is added has the same concentration of cellulose, and what is done is to add enzyme. In the graph you can see that the reaction takes place over time, decreasing the amount of glucose obtained. From 75 minutes onwards the reaction rate decreases as well as the amount of glucose obtained from the reaction, which is due to the fact that the substrate —cellulose— is being depleted.