Complex sentence: Records cannot identify exactly why the Roma migrated from India; there are a number of possible motives, famine and possible army invasions assembled by Alexander the Great and/or Mahmud of Ghazni.
Split into simpler sentences: Records cannot identify exactly why the Roma migrated from India; there are a number of possible motives. Famine, and possible army invasions assembled by Alexander the Great and/or Mahmud of Ghazni.