im sure you may have been going a little faster than 15-20...but [censored] tires or [censored] roads combined with rain makes it easy to lose control...especially when slamming on your brakes...my car jerks to the side too when i slam hard on the brakes...so combined with rain i can see how that could happen.
unless you were really going 15-20...then i can't defend you there

atleast nothing got hurt