I remember stories in 2015 about Trump conferring with the Clinton's about running. Scuttlebutt at the time, IIRC, was that Trump was just running to take out the best pub candidates so Clinton would have a patsy opponent in the general.
Rumors aside, and looking back with what we know now, maybe those talks were the beginning of the sting. Trump chummed the Clinton's with a run and they took the bait. I have to think the Clinton's would have a team out to take down any potential opponent, dem or pub, so they assigned a team to Trump and Trump and the boys were waiting for them.