I think they're in the tank for ratings, which means they're in the tank for a close race. Which means that, since in any rational world Trump would have been dismissed long ago, they've had to build him up (with obligatory negative stories when they couldn't possibly be denied) and tear her down. That's been the M.O. for pretty much all the news stations. But arguably NBC built him up as a "plausible candidate" before the others did, due to his long association with them.
And that has been the worst part of this election cycle for me. Trump skated through the primaries because he had so much free press coverage. But it was a symbiotic relationship as the news stations wanted him on there 24/7 because people tuned in for some reason (probably for the same reason we rubber-neck when we see car accidents). So it's probably not a stretch to say the news stations are, at least in part, to blame for Trump's ascendancy with NBC at the top of the list. Ratings mean money so the news stations are going where the money is, no matter the consequences. I think we are witnessing another dark side of capitalism.