Banksy posted a video of it being made. He says he put it in in case it was ever auctioned. It is believed it was activated by someone he sent to the auction using a mobile phone.
https://www.mirror.co.uk/news/uk-new...t-how-13373762
I don't believe for a minute that an auction house of that nature would expose themselves to this sort of publicity, they weren't even sure of the legal status of the bid once the item was snafu'd