There is a widespread practice not to wear tallis and tefillin for the morning prayer on Tisha B'Av, instead putting them on for the afternoon prayer. There is also a prohibition against wearing tefillin after sunset on any day. In congregations where mincha and maariv are prayed together, everybody removes their tefillin after mincha before sunset.
In my experience, everybody also removes their tallis before maariv of motzei Tisha B'av, despite the fact that there is clearly no prohibition to wear a tallis in the evening. The tallis gadol is worn for maariv on Yom Kippur and, in my experience, most people also leave it on after ne'ila and through maariv of motzei Yom Kippur. Furthermore, the general practice is not to remove the tallis katan in the evening on a daily basis.
So why is the common practice to remove the tallis gadol for maariv after Tisha B'av? My guess is that people do it for no particular reason other than because they're already removing their tefillin. Is there some other reason why the tallis should be removed?