The energy the Doctor was exuding (regeneration energy, possibly combined with the last of the vortex energy leaving him?) attracted the Sycorax to Earth. But the pilot fish parasiting of the Sycorax were also drawn to the energy. They reached the Earth just ahead of the Sycorax ship and used a disguise when they arrived, locking on to the prevailing cultural event of the time - Christmas. The TARDIS has a chameleon circuit - the pilot fish must operate along similar lines, the better to reach their target relatively undetected.