It's just randomly happened again.
I see it as a weird act of vandalism to block someone's screen intentionally such that the only way to get rid of it is to close the browser and start again, but not knowing when the next time will be.
I obviously need to get out more, but I am fascinated by the modern techniques of advertising and the psychology that must be involved in thinking how can I force someone to have to sit and look at my ad. It's a bit like those people who kidnap people by stealth on holiday and make them sit for two hours watching a hard sales pitch for a Time Share. 