Usually pile caps have 2 or 3 piles below one pile cap. And then from the pile cap the column is erected ... if we keep in mind the eccentricity of the pile is not much and load from the column is transferred equally and at centre of pile cap. Then the pile cap while resisting the load from the soil and piles will bend or move upwards . Causing tension in the bottom of the pile cap. Hence the main reinforced will be at the bottom as this is the tension zone. Also punching shear effects have to be considered.