I recenly had a DE back into the pool problem. I just watched the return very closely, there was no mistaking the DE coming back into the pool from the return. Turned out to be a hole in one of the grids and this was a brand new hayward DE 60 filter. I had to totally dismantle the filter to find it though, it wasn't easy to spot until I took it all apart. Also try and watch the return right as you turn on the pump as mine usually stopped after 5-10 seconds.

You mentioned 1 1/2 HP pump but which hayward filter do you have ? It's probably oversized just like mine is and you are probably exceeding the flow rate. I am going to change impellers to reduce flow rate. Here are the general culprits for DE back into the pool in no particular order

1) hole in fabric covering grids
2) crack in manifold somewhere
3) pump oversized
4) check the fabric around the air relief valve, make sure it's tight and without holes.
5) faulty multiport valve not seating properly, try moving the handle around and see if it helps (while pump is off)
6) check the outlet o-ring

good luck