When you join the (UK) Post Office, a government owned and run organisation back in the eighties and through the nineties (with unique powers and its own investigation branch, police, like for the railways) you have to sign the Official Secrets Act.
The 1989 revision of the Act introduced provisions surrounding illegal disclosure of sensitive information – otherwise known as leaking. The maximum sentence for espionage under the Official Secrets Act 1911 is 14 years, but longer terms can be imposed for a series of offences.
The problems with Horizon were known from day one, but who is going to risk their: bonus, job, or even prison? Who thought it would get so out of hand? Errors in the system were corrected remotely overnight, with the best intentions, but one change can generate knock on effects. The Post Office Investigation branch (police force) were there to arrest "imagined" criminals and were the people who dealt with mail theft etc. with absolutely no knowledge about complex computer systems. It's not too difficult to imagine how a series of events led one to another and finally a catastrophe that destroyed people's lives and even killed some. A good analogy would be a series of aircraft crashes which are found to have been caused by a design fault, the only difference is the scale and the cover up!