Ditto, I reckon it must have been diesel. Roundabouts are buggers for it, though usually on the roundabout itself rather than an approach road. In the wet, there's often a tell tale rainbow slick to be seen but as you seemed to be riding into the sun, the glare would probably have stopped you seeing it.
The white road markings/lines can be similarly dangerous in the wet, but you were well clear of those so that wasn't it.
I've come off in ice (twice) and that's similar. The bike just goes on you, and there's nothing you can do about it. It's pure luck if you stay upright.
It's sadly one of the inherent dangers of biking and I'm just pleased you're okay.
