This might just be reporters drumming up talking points, but I have heard Capella might be available from multiple places now.
Why would the Rockets trade him? In short, if they think they could fill most of his production as a rim runner through a cheaper option, they could trade him away to try to get a lesser rim runner and more depth.
They desperately lack depth right now. They are giving McLemore, Sefalosa, Rivers, and House a lot of minutes right now. Those guys are end of the bench players at best.
The Cs almost have too many guys right now. They could trade Ojeleye or Green if the Rockets liked either of those cheap depth options. Theis and Kanter would probably have to be included for salary purposes (or Smart).
A potential deal could be Kanter, Langford, and Williams for Capella. Or Kanter, Langford, and Ojeleye for Capella. Or Smart and Williams for Capella.
I wouldn't do this deal, because I think Williams will be at that level one day--maybe soon. But still, a trade like that could work.