I reckon it's the sun that's woken them from hibernation.
I saw no fewer than seven yesterday, including one making a very nice noise as he powered past my 60 or so on a B-road on the way to work and ending with the one on the back of a recovery truck that clipped my wing mirror while squeezing past me on a single-track road (me stationary and pulled over, him doing about fifty - idiot!).