what would cause my ABS light to go on AFTER the wheels start to roll?

[Obin Olson]

like the subject what would cause my abs light on the dash to come on after i start the car and roll the wheels...I can sit all day long before I start moving and the light will stay off but about 3mph and the light comes on...if I stop at a stoplight and push the abs switch then it will go off till I start moving again...ideas?

TIA

-Obin Olson
88 90q