We had Christmas on the Wirral this year. Alas, no high tides with spectacular numbers of waders but thousands of gulls and I did come across interesting articles on yellow-browed and Pallas warblers on the Deeside birds site ( http://www.deeestuary.co.uk/news.htm#first). They draw on British Birds (White & Kehoe), published in February 2016. The YBW normally migrates south-east to Burma, Thailand, Vietnam, south China area and as far south as the Malayan Peninsula. The evidence is that Yellow-browed Warblers are flying on a great circle route starting exactly 180º in the wrong direction- this is classic Reverse Migration - rather than this being a new migration route.
"We don't know what causes this Reverse Migration (genetic defect? environmental effect?) and we certainly don't know by what mechanism Reverse Migration is causing more and more birds to fly the 'wrong way' each year."
The articles also speculate why they don't seem to be over-wintering successfully.