How to brake without falling during wet weather?
when I pressed the rear brake lever, the wheel stopped turning immediately, despite the slight pressure


if it locked up with only slight pressure, either your brakes lack modulation, or it had really poor traction.

The first (modulation) is generally adjustable.

The second depends on your tyres, the road surface, possibly oil/petrol/diesel spills, loose material etc. etc.

Since you don't think there was loose material, and say the traction was fine previously, an oil spill is certainly possible - look out for those rainbows on the road surface.


  Worse, the bike didn't stop, it started to accelerate


If the back wheel is locked, skidding it should still have some braking effect, so I'm inclined to view this as evidence for poor traction.


  I assume lowering the saddle would've necessitated much more effort from the front brake to throw me


Did you go over the handlebars, or just lose grip with the front wheel and crash?

If the former, which I think is what you're suggesting - lower saddle might help on steep downhills, but generally you need to brace yourself down &amp; backwards with your arms. The goal is to stop yourself sliding forward relative to the bike as it slows down, and also to reduce the angular moment about the front axle by keeping your weight low.

Interestingly, this does suggest there was no traction problem with your front wheel, so something happened to impair traction at the back only.


  My bike is suited for XC racing


Does that mean it has knobbly tyres? More contact area is generally better on asphalt, so slick or textured tyres will have better traction (and, as a bonus, generally lower rolling resistance).


  Should I have kept the rear or both brakes engaged constantly


I'd usually drag the back brake to control speed on descents - to avoid overheating, don't do this continuously, but pump it and ease off to let it cool. Alternating with the front will also give the back brake a chance to cool - but note the front brake affects steering more, so definitely keep braking to the straights as much as possible.