Many people, myself included, have been saying this for over a year now. The MSM was giving him all the attention after he announced, they were even going live to his speeches when he started out. We know they are slanting polls now and were no doubt slanting them early for Trump. They helped create the snowball that kept growing and was eventually irreversible.
The tape of Trump talking dirty was the final proof for me. The Clinton camp has been holding that tape for a long time and if they really thought Trump was a threat to beat her, they would have released it early in the primaries and Trump would have be done.
I think the term is “hoist with one’s own petard” .... they wanted Trump, they got Trump & then realized they had totally and completely underestimated the guy. :-)