Al-Haafith Ibn Hajr, may Allaah have Mercy on him, said in Fat-hul-Baaree (2/407):
(1) It has been said that the wisdom in the imaam turning to face the followers is so that he might teach them something that they need to know. So then based on this, this act of turning around is only for those who are in the same situation as him (sallallaahu 'alayhe wa sallam), those who intend to teach or give an admonition. 
(2) It has also been said that the wisdom in it is that those entering the masjid will know that the prayer is over. Since if the imaam would have remained as he was, people may think he is still making the tashah-hud for example.
(3) And Az-Zubayr ibn Al-Muneer said, "The imaam having his back to the people is only due to the right of his leadership, so when the prayer ends, this reason vanishes. So then to face them at that time will illeviate him of any pride or arrogance he may have towards them, and Allaah knows best."
 This is refuted by the fact that the Prophet (sallallaahu 'alayhe wa sallam) did not used to teach them or admonish them after every prayer himself, rather he would set times for that so as not to overburden them.
******************** سبحانك اللهم وبحمدك أشهد أن لا إله إلا أنت أستغفرك وأتوب إليك