You will one day, happens to everybody one day, happened to me once right in front of a packed bus stop in Nottingham city centre, so embarressing.
I've only ever felt the back end go when applying more power. Something i don't do on roundabouts, usually take them with no power or constant.
I used to ride all weathers, never came across slippery roundabouts, luckily. Looks like this guy must've hit a foreign liquid on the road surface or something.