I guess it's mist/fog. It was forecast. The wind direction suggests that 09 is the active runway and it has no Cat3b capability, unlike 27. Thomas Cook from Antalya also shown as diverted to CWL. Several aircraft seem to have landed since the diversions so if fog is/was the culprit it is presumably capricious this evening.
Currently (2000 hours) three more aircraft holding (two Ryanair one easyJet) according to FR24.