For the accurate version, see the posts by Hoverman and Heliport.
The offence of Endangering safety of any person or property under Article 64 of the ANO 2000 can be committed in two ways: either "recklessly" (serious) or "negligently" (less serious).
In a nutshell, if you take care to recce the site to ensure it is safe to land, but fail to a hazard, the law is that you are negligent - even if you've done your best to exercise great care. Taking care is not enough - genuine mistake is no defence. (eg A driver pulling out from a side road into a major road who looks before pulling out, and is convinced the road is clear, but fails to see a motorcyclist is, in law, negligent - even if he's done his best to take great care.)
The CAA set out to prove that the pilot was reckless. The alternative 'negligence' charge was there so that, if the jury was not satisfied the pilot was reckless, it would still be open to them to convict him of being negligent. This gives the prosecution 'two bites at the cherry' but it is allowed.
NB: The pilot never denied that what he did endangered people/property. [Whether the lady fell backwards in her chair, or was lifted up 5 feet and blown through the air for 20 feet (!!!) was completely irrelevant to the charges. Blowing tables/chairs around would have been enough for this element of the charges.]. .The only question for the jury to decide was whether he did so recklessly/negligently.
I repeat, the CAA's case was that the pilot flew recklessly. However, on the third day of the trial, after the versions given by the prosecution witnesses had been tested in cross-examination, the CAA agreed to drop the more serious 'reckless' charge, if the pilot pleaded guilty to the less serious 'negligent' charge.
Sadly, the CAA dropping the more serious charge didn't make a good story for the Press/TV.
Warning:. .Even if you do your best to fly safely, and follow the normal procedures to ensure your landing area is safe, you are still liable if you fail to see a hazard.. .That may seem very harsh in the context of criminal offences, but it is the law.