This was the article which caused him to freak out: http://entertainment.inquirer.net/216210/tom-hiddleston-takes-break

It was then picked up by the Daily Mail which implied he was taking much of 2017 off (no lies there by the looks of things!).

However something we have learned from the last year is that when TH ' slips are moving while he is talking to the press, he is lying. For example, about smoking, about what he previously said about TR being Loki's last outing, and many other examples both big and small.

As to why: I think it started as a mixture of things: him being super controlling about his image/career and wanting to say the "right" thing, having on occasion let slip things he shouldn't and having to row it back, and being a bit political about things we shouldn't necessarily expect him to tell the truth about, even though he appeared to be very open (whether he has a GF). What it has become (and I see this a lot as a lawyer) is lying as a habit, when it isn't necessary.