#1 United fans’ answer
There was once a time when Liverpool reigned in England. For the ones that are new to football: yes, this is true and not a glorifying fairy tale that you read in books. Liverpool, not Manchester United, were the kings of England—and like most kings, their fans were full of themselves.
So in a bid to deride their fiercest rivals, some Liverpool fans held a banner, asking their rival fans to come back when their team have won 18 Premier League/Football League titles. Needless to say, Manchester United did win their 18th title when they clinched the league in 2008-09 after beating Liverpool by a superior goal difference.
And then, it was the turn of the Red Devils’ fans to display some banter—and they did it with this banner.