I think the nature of the NBA and basketball in general lends itself to trades. With relatively small rosters and guaranteed contracts trades are the major way franchises change. When teams want to make changes IE rebuild, reload, or add pieces to their core they have to have cap space or trade-able assets.
The Celtics are particularly targeted in rumors because A. DA has a history of being active on trade market. B. Has an unbalanced roster. C. Needs to clear up roster space. D. Are terrible in their current construction.