In a recently published study calles "the role of digital in TV", Google seems to prove that TV related activity on Youtube increased heavily over the years. They analyzed Google and YouTube searches (remember: YouTube is the second largest search engine in the world, right after Google), engagement and actions and it seems logical that people search for TV shows on these platforms - where else? On the other hand, you could expect that for 7-day-catch-up and additional info they would use the TV stations' websites (and to some extent, they find them through Google).
But why this remarkable growth dynamic? The study unfortunately only breaks down desktop/tablet on one hand and mobile on the other hand. My suggestion is that these millions and millions of people who own an Apple TV, Chromecast, Fire TV, Roku, Boxee or whatever else might hook up to TVs or is built-in (like Smart TV) always have YouTube at their hands. Reliable performance, and it always works with every hardware. Maybe this TV station is not integrated on your hardware with an app or maybe that TV station's iPad app does not feature Airplay... but you can always be sure that YouTube will. If you want to stream TV content to your TV, YouTube is the choice, and since these TV-connected grew significantly over the past years, this may explain the growth of TV-related activity on YouTube.
So, TV marketers will have to not only get their distribution straight but also think about using YouTube as a promotional channel for their content - or even as a primary digital distribution channel. There's one more finding that might interest them in this research [quote]: "Search, video and engagement activities, which show a positive correlation to viewership, can provide additional insight into a show's popularity." The big money, at least for TV stations, is still in broadcast advertising. This briefly mentioned "positive correlation" is what they are all looking for.