Why did Garnacho celebrate his goals by sitting on the billboard?

Manchester United beat West Ham 3-0 in the Premier League matchday 23 fixture at Old Trafford on Sunday, with Alejandro Garnacho scoring a brace to help his side climb to sixth in the English top-flight.
The Argentinian winger celebrated his two goals by sitting on the stadium billboard with his teammates. Rasmus Hojlund, who netted the opener to put his team ahead, joined him in celebration. But why did the 19-year-old celebrate his double like that?
The Red Devils visited the London Stadium in the Premier League matchday 18 fixture on 23 December 2023. The Hammers managed to beat a dispirited United side, with Jarrod Bowen and Mohamed Kudus netting in the second half to give their side all three points.
The Ghanaian midfielder, who joined David Moyes' side in the summer transfer window, sealed West Ham's victory in that game by scoring in the 78th minute and celebrated by sitting on a billboard at his home stadium.
As a result, Alejandro Garnacho imitated his celebration to mock his opponents when they were losing. The 23-year-old didn't take too kindly to his celebration being imitated as a taunt. But such imitations are certainly becoming popular in the Premier League.
Brentford striker Neal Maupay also imitated James Maddison's celebration in his side's 2-3 defeat to Tottenham Hotspur when he broke the deadlock in the 15th minute to give the Bees the lead. This kind of taunting of opponents could become a trend in England.