This sounds like the ABS pump/modulator is developing sticky valves perhaps. Braking hard activated the modulation circuits and cleared (for now) what was playing up.
OK, this is inductive speculation, based on this question I posted in this forum. However If you do not know the age and condition of your brake fluid, it would be an idea to flush and change the system.
OK, this is inductive speculation, based on this question I posted in this forum. However If you do not know the age and condition of your brake fluid, it would be an idea to flush and change the system.