Thanks for the responses. Upon further reflection, I think I know what is happening.
The original PB did not use any check valves in the system. I have a spa that is higher than my pool. When I shut the pump off the water tries to equalize and pushes backwards from my spa return through my pool returns and main drain/skimmer resulting in the DE in my pump. Sound possible? I bought a check valve prior to noticing this problem, just haven't installed it yet.
In the morning the pump basket is still full of water so I don't think I have a leak. Also when I installed the filter I literally weighed out the DE. I have heard of adding 80% of the suggested amount though.