Why was he given a police caution ? He did nothing illegal, nothing that police had previously been asked to be told about, so why a caution ? Yes what he did accidentally caused some disruption; but this was not intended.
They probably mixed up the Simple Caution (that is a form of mild wrist slapping) with the warning, also called a caution, that the Police give you before talking to you about anything in which you could by any stretch of the imagination be considered a suspect or witness. The equivalent of the Miranda Rights in the US.
"You do not have to say anything, but it may harm your defence if you do not mention when questioned anything which you later rely on in court. Anything you do say may be given in evidence."
When I was in the (UK) cops we were trained to dish this out to anybody we spoke to even semi-formally. If nothing comes of your conversation then no problem; if you fail to caution somebody prior to speaking to them, and then their testimony becomes a big deal, the lack of a caution at the start can be a case-breaker. So you basically say it to everybody to be safe. But it is NOT something that is recorded (other than in your pocket notebook) or otherwise appears on anybody's record.