Could be lots of reasons; new country, new league, sat on the bench, sick of Pep's tactics. I don't know exactly but there has been more than one story printed about it this season by the media (Makes me think someone somewhere is talking, of all the players they could talk about they chose Cancelo) and I think even a few people on here have dropped a few hints that all is not well where Cancelo is concerned.
The deal for Semedo looked to make sense but Juve's deal puts more on the table for Barca so they will take that.