What could cause the pressure to rise within 30 minutes in a DE pool filtration system?

This is normal with my DE filter. When I first turn the pump on the pressure starts at about 5 psi. During the next 30-60 minutes, it rises to 15 psi and then stablizes there. So, I use 15 psi as my "starting" pressure and don't backwash until it reaches 20 psi.

When was the last time you had the filter broken down and cleaned????