When your beer is actively fermenting the yeast is producing CO2 bubbles and they stir up all the trub. Once that slows the trub can start to settle. That is what you are seeing. It could have happened in the primary fermenter had you left the beer there and you would have avoided the possibility of oxidation of the beer or contamination. Beer kits often have instructions that tell you to move the beer to secondary. Throw out those instructions or at least ignore the part about moving to secondary. Very few beers will improve with secondary and very many of them can degrade by doing so.
