Hi rutineli!
The strangest thing is that they seem to do it during the highest temperatures.
Last year I saw a Song Thrush in the middle of a field, wings as you've described and beak open, just laying like a statue. He was still there a half an hour later....in the boiling sun!
Dust bathing is fairly common for removing mites and other nasties. 'Anting' is fascinating to watch as the birds wriggle in what appears to be ecstasy as the formic acid supposedly kills the bugs ...but what 'sunning' does has yet to be proved. Killing mites again, perhaps, although I prefer your ' stoned' theory!