What causes "dieseling"

I began having a dieseling-upon-shutdown on my stroked Hemi. This was low on the list of tuning issues but it didn't start occuring until the move to FL. I initially thought excess fuel was puddling on the floor of the cross ram, causing the issue. Long story short, I was using the 93 octane at various stations with price per gallon in mind. I recently switched to Top Tier only fuel and the issue is gone. I do the water trickle procedure about every 3 to 6 months as well. Hope this helps.