1.
Be the determining factor in; cause. For example, The heat wave accounts for all this food spoilage, or Icy roads account for the increase in accidents.
2.
Explain or justify, as in Jane was upset because her son couldn't account for the three hours between his last class and his arrival at home. Both of these related usages are derived from the literal meaning of the phrase, that is, "make a reckoning of an account." [Second half of 1700s]



