D.E. coming up from drain overnight.

Should not be possible. It would have water going backward through the filter and then through the pump and then to the pool.

Although I did have DE just come up my drain when I was adding DE and forgot make all the suction come from the skimmer (instead of part from the wall). The heavy slurry was going down the drain pipe in the skimmer and coming back up in the pool. Just put the diverter back in to all floor and sucked it back up.
 
Probably from air in the filter. It is compressed by pump pressure and expands when the pump shuts off and can push water backwards through the filter carrying the DE with it.
 
Also it could be that there is a leak somewhere in your system. When you turn your pump off you can probably hear water draining out of the filter, and it ends up falling through the plumbing back into the pool taking some d.e. with it. Finding whats causing the leak is sometimes obvious, but sometimes it can be a challenge
 
Air is most likely getting in from the waste port of the multiport, or the air relief on top of the filter. If it's from the waste port, then disassembling the multiport to check the gasket might help. When reassembling the multiport, lubricate the o-rings to help the multiport seal better.

You could plug the waste port until it's needed for backwash. Adding a check valve between the pump and filter can also be helpful.
 
Found the source..
During cleaning i noticed the plastic piece on top of the filter was loose , so i tightened it up and it broke ...
Went to Leslies got a new one, did not overtighten it..
It's a bad design part.. The seal is not very good ...
I added some G.E. silicone II , and no more D.E. coming back out of drain...
 
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