There are many reasons for hematocrit to fall below the normal range. Several reasons are internal bleeding, overhydration, sickle cell anemia, anemia, malnutrition, and cancer; although there are many more. Hemodilution would be a reason for decreased HCT.
One example is during pregnancy even though the blood volume increases, and the red blood cells increase, plasma increases more, diluting the blood, resulting in anemia, and decreasing both hemoglobin and hematocrit levels. The lack of erythropoietin hormone, which produces red blood cells, can cause anemia which would lower HCT levels.