It's a dirty wheel speed sensor. The faster the ring spins, the weaker the signal through the wire becomes, due to magnetic fields and cool physics nerd stuff like that :P
The signal normally weakens some at higher speeds, but not to a point that the signal doesn't get through. A dirty sensor compounds the situation and makes what would be an ok signal too weak for the computer to detect.

The ABS sensor freaks out when the pulse ring signal disappears from a wheel because it's programmed to read the lack of pulses as a wheel that isn't turning. The freak-out throws an ABS light and it resets when it works normally again at lower speeds.

A can of brake cleaner and a good spray on the sensor and tone-generator wheel should solve the problem.

Good Luck