Pro-Agreement Leslie Cree was one of three candidates hoping to capture the party's nomination.
At the last minute on Wednesday night, Mr Cree withdrew from the contest citing personal reasons.
It has now emerged that days before the meeting, an anonymous letter was sent to him threatening to reveal details of his personal life.
It is understood he told Wednesday night's constituency meeting someone had used underhand methods to try and force him out.
In an anonymous letter, sent several days ago, a reference was made to the fact that, 36 years ago as a young unmarried man he had fathered a child.
" Lady Hermon has my full support and I will be helping her win the election in North Down "
The child was put up for adoption.
Mr Cree told delegates that his wife and sons had long been aware of the situation.
The delegates then gave him a standing ovation. But it is thought he decided to withdraw from the contest because the outpouring of sympathy may have given him an unfair advantage and he did not want to win the race on those terms.
In the end Lady Sylvia Hermon, who is pro-Agreement, beat anti-Agreement unionist Eric Culbertson by 112 votes to 29.
Mr Cree has released a short statement on what happened to him.
He said: "Lady Hermon has my full support and I will be helping her win the election in North Down."
Meanwhile anti and pro-Agreement unionists from North Down were united in condemning the blackmail attempt against Mr Cree.
Anti-Agreement Peter King, who says he will leave the UUP over the deselection of Mr Weir, said it was "an appalling attempt to blacken the name of a man whose integrity was beyond question".
Pro-Agreement Alan McFarland spoke of "appalling tactics" against an honourable man.
But he said the incident was also an attempt to blacken Lady Hermon's team of workers because some anti-Agreement people were trying to suggest her people were behind it.
There has been a long-running wrangle within the party over who should fight the general election seat.
The original choice was anti-Agreement Peter Weir but he was deselected after an internal disciplinary hearing suspended him from the UUP for voting against the party line.
The issue has been an embarrassment for David Trimble illustrating the deep rifts between anti and pro-Agreement unionists in the party.
Lady Hermon will be challenging the sitting MP Bob McCartney from the UK Unionist Party.
The SDLP candidate Marietta Farrell and Stephen Farry from the Alliance Party are also contesting the seat.