Well, when you did your partial mash you did steep your grains in a bag right? If so the fact that it's a partial mash shouldnt affect you too greatly. The most likely thing that happened is that on bottling day when you moved your fermenter you shook up your yeast/hop trub that had settled to the bottom, then when the beer in your bottles settled the suspended trub just fell to the bottom.
On bottling day I usually set my fermenter up onto the counter then let it settle for at least a half an hour while I clean and prepare my bottles. By the time everything is cleaned most of the particles that got kicked up while I moved it from my basement have settled out again and the beer that gets siphoned to my bottling bucket is mostly clear.
I then let that sit for 10-15 min (with a cover on of course) while I wait for more of the sediment to settle out.
The amount of trub can be influenced by a number of factors from the OG, the FG, to simply the strain of yeast used.
