Rewatching the MotD2 highlights on iPlayer (
http://www.bbc.co.uk/iplayer/episode/b06h8b8n/match-of-the-day-2-201516-04102015) and Dier gets caught pressing high up the pitch after Rangel and Fernandez do a quick 1-2 which by-passes Chadli plus Eriksen.
So when Ayew receives the ball on the halfway line he's in plenty of space with Alli and Dier both 5 yards away. Alli then sprints back to track Siggy' run into the box leaving Ayew to jog in unmarked, Dier only arrived back in the area as the ball is flying into the net.
Alderweireld initially looks like he would go and help Walker cut out the cross but then he notices Ki running down that flank and ends up in no man's land.
Perhaps Dier could've busted a gut and prevented Ayew from having a free header but, as you say, it's a classy attack by Swansea; just the sort of thing I hope to see plenty of from us once Son returns from injury to replace the ponderous Chadli.