If the rain had just moved in, the runway surface could well have been more oily than wet.
The top part of pavement is full of petroleum-based products, mostly microscopic bits of tyres. When it first rains after a dry spell, those oily bits of petroleum float on the surface of the just-fallen rain, and that is when a paved surface is the most slippery. This is particularly true on those areas where the tyres are accelerating, decelerating, or turning (because thet's where they shed the most rubber). After it has been raining for a few minutes, the water/oil ratio grows large enough to eliminate the oil surface, and it's just a question of a wet surface.