Why does there tend to be more call and response in Sephardi communal prayer in comparison with Ashkenazi minyanim e.g. saying pesukei d'zimra in unison. Also, during the repetition of the shemoneh esreh some Sephardim have the custom of repeating words along with the shliach tzibbur or adding extra 'call and response' phrases to kaddish.
What is the reason for doing this and from where did this develop?
Not sure about the call and response you mention but in general the Sefardi tzibur is more inclined to be yotzeh with the shliach tzibur as apposed to Ashkenazi tzibur where everyone is motzei themselves. See biur halacha siman 59 d.h. binachas, (you will find that much of the confusion about whether or not to say amen to shatz by birchas krias shema is unwarranted. Ashkenazim are not beying yotzeh from the shatz and have no reason to say amen). Also see tshuvas chassam soffer orach chayim in the middle of siman 17 where he too discusses this difference between Ashkenazim and Sefardim as related to his subject there.
I have no evidence to support this theory, but I believe that it is in order to ensure that the prayers are articulated carefully and correctly. Muttering ("watermelon, watermelon, watermelon,...") is discouraged.
