Reports that they may have been released for a ransom or trade to release Boko Haram men.
Via Daily Mail:
Overcome with emotion following their dramatic release, the kidnapped schoolgirls who have been freed from their Boko Haram captors in Nigeria have been pictured for the first time.
More than 200 girls were abducted from a school in the village of Chibok in April 2014 by the jihadi group prompting international outrage.
However, more than two years on, 21 of the captured teenagers were released in what is thought to have been a swap for jailed Boko Haram leaders.