ABS playing up

[squizzy28]

been a while since i've been on here but need some help with my parents AU 3 fairmont. They recently got new tires, pads and rotors put on and ever since then the abs has been randomly kicking in. it only happens occasionally 3-4 times a drive but it happens in weird spots like stopping at a traffic light or pulling in to a drive way. no hard braking. it feels like the drivers side front and you can feel it through the brake pedal and hear it. no lights come on the dash and our mechanic has but it on his diagnostic computer and no errors come up and he can't figure out what it is. he tried to fix it by again changing the brake pad and rotor and cleaned the sensor but made no difference. anyone have any ideas??