Water on the pads/discs then - gets between them, in the same way as when a car aquaplanes due to water getting between tyres and the road.
It soon burns off or gets squeezed off by the pads, it's just the initial brown trouser moment you had while it did it! lol
That's why you sometimes see "Try your brakes" signs after fords - so you get rid of the water in a safe place rather than just before the next corner or the junction you need to actually stop at and not sail across lol