Ms Horne organised for an accountant - trust member Moira Fox - to undertake her duties as a contractor in her absence.
But when Ms Horne returned to part-time work in October, she discovered there had been no reporting to the board and the annual accounts were not ready for audit, as she expected.
The authority said Ms Horne was left with a "bewildering" amount of work to do, which compounded the pressure on her.
She was diagnosed with anxiety and depression in November and again went on full-time sick leave.
Authority member James Crichton said there was conflicting evidence over whether Ms Horne had asked Ms Fox to do the audit in her absence, but there were clear problems with other work that both parties had agreed needed to be done.
The trust's "extraordinary" decision not to address those problems had contributed to Ms Horne again becoming unwell, he found.
"The trust, in failing to address the deficits it identified in the performance of Ms Horne's role while she was on sick leave, exacerbated the problem that she faced on her return to duty and actively contributed to making her unwell again."
Ms Horne returned to work last March but was made redundant later that month. Her temporary replacement, Ms Fox, was hired as a contractor to carry out her duties.
The authority found the decision to disestablish Ms Horne's position was not genuine because the tasks she did would continue to be carried out by a contractor.
It also found the trust did not take proper steps to address concerns that it was seeking to advantage own of its own board members, Ms Fox, over a staff member.
Ms Fox was present at the March meeting at which the redundancy decision was made, but did not declare her interest in the matter - despite putting in a proposal to take on the contractor role the previous month.
The trust also consulted Ms Fox over Ms Horne's feedback on the proposed redundancy.
"Plainly, it was completely inappropriate of the trust to allow Ms Fox to be in any way involved in the decision to dismiss Ms Horne for redundancy," the authority noted.
It found Ms Horne had been unjustifiably dismissed and ordered the trust pay $8000 compensation and $25,000 for lost wages. Costs were reserved.
Ms Horne could not be contacted. The trust did not respond to a request for comment.