Nothing magic about this. These asteroids probably hit during the late, heavy bombardment. According to the Nice model, this is a result of Jupiter and Saturn being in an orbital resonance (Jupiter orbiting once for every two times Saturn orbited). A number of scientists (notable among them, WF Bottke), speculate that the Hungaria family of asteroids was nearly completely depopulated that this time as most were knocked into the inner solar system.
A recent discovery of an 100 km diameter, 2 bya asteroid crater in Greenland is the first direct evidence of the bombardment hitting Earth. Weathering and continental drift have made getting such direct evidence difficult.
Examination of isotope ratios in comparison to what is known for for various types of asteroid are somewhat inconclusive for a particular family being the exclusive donator to Earth's oceans. But, I stand by the estimation that the Hungaria family provided the major portion.