Increasing memory usage after local backup

Hi,

I'm experiencing a very strange situation with my Plesk.
Every night my Plesk panel runs a complete backup of the server and, after that backup, the total memory usage starts increasing for all over the day up to the next backup. What warns me is the fact that every night the basal level of the memory usage increases.

I checked the ram usage and it seems that this increased memory usage is due to an increase in cache usage.

Do you have any suggestion?