What makes coffee grinds sink in a french press?
The question should really read "What makes coffee sink?" It does not matter if it is in the form of grinds or beans (though it is easier with grinds) nor does the receptacle. The issue is buoyancy.


  buoy·an·cy  (boin-s, byn-)
  n.
  1.
  a. The tendency or capacity to remain afloat in a liquid or rise in air or gas.
  b. The upward force that a fluid exerts on an object less dense than itself.


As the coffee is soaked in the hot water chemicals are extracted from the grind, mostly oils, that have an impact the buoyancy of the coffee. Once, the desirable chemicals are extracted to the surrounding water (again, moistly an oil) the buoyancy of the coffee is altered and the residue sinks to the bottom. 

During my time in the U.S. Army field coffee was made quickly by getting a 15 gallon stock pot boiling and just pouring a five pound can of coffee in. Allow to boil for 5 minutes, reduce the heat and hit the side the stock pot with a ladle. Immediately all (ok, most...) of the coffee would sink to the bottom and fresh (if not altogether worthy of Starbucks) coffee could be ladled out. Why whack the pot? This would disrupt boiling of the water and allow the coffee to sink quickly.

Why are your grinds sinking? Well, the best guess is that they came deficient in the compounds that make the coffee normally buoyant. If the brand/roast is not up to your satisfaction you should be able to return it for fresh. If this a brand to which you are accustomed, but this batch isn't 'normal' then it is probably an anomaly, as you said the coffee tastes normal, so I wouldn't not worry to much about it.