A couple armchair-quarterbacking notes:
-you said the system "locked up". ABS never locks the wheels. When abs engages, it is QUITE noticeable: you feel it in the brake pedal and you may hear/feel a pulsing "shudder" throughout the vehicle.
-you didn't say how fast you were going or how hard you were braking. As other posters have commented, the engine does a good deal of "engine braking" and it is very evident when very gently slowing. When the vehicle downshifts it can feel like a decrease in braking. If this resulted in a collision you are following way too close to the car in front of you!
-The human body isn't very good at interpreting changes in the change of momentum. For example, a change from a "hard stopping" deceleration to a "gentle stop" deceleration can actually FEEL like an acceleration. Thus your "lurch foward" you felt was most likely a change in the slowing speed.
-Hitting a slick spot will decrease overall braking to the wheels slightly. As mentioned above, this decrease in braking can feel like an acceleration. (fun fact: engaging ABS on a motorcycle *REALLY* makes this feel evident!! It is quite a strange feeling)