Some good speculation here and some bad. In defence of the pilot I wonder about the elephant in the room and that is aquaplaning. 9 x the square root of the tyre pressure. I have only flown the 200/300/400. On the 400 we ran the tyres around 220psi. This gets very close to a common Vref. Add water and no grooving and a little tail-wind for good measure. I have flown four different Boeings and the 737 is the only one I have experienced aquaplaning and it very nearly was my undoing once. A strange sensation and a horrible feeling of accelerating.