Anti-lock Brakes

I have a 98 Sierra 1/2 ton 4x4 and I was driving around in the snow today and for some reason the abs kicked in while I was braking. I had light pressure on the brake pedal, I was just easing it on and the abs kicked in.
Is this normal for driving in slippery conditions?

Also, what determines whether or not the abs kicks in?

ABS is activated when the vehicle speed sensor (vss) senses the wheels are locking up under braking conditions. There is a motor or pump that pulsates as you would on an older vehicle that doesn't have abs. The vss sends a signal that the wheels are locking up to the abs controler or bcm and it tells the abs pump to activate. Hope this helps.