Why did my emergency braking system trigger for no reason?

I was driving at 60mph on an A-road, the vehicle in front was 150 yds ahead also doing 60mph. Suddenly without any warning the emergency braking warning light flashed on, the sound bleeped and the brakes...

Is a brake warning light an MoT failure?

I was told last year that the Mazda SBS light being on and not functioning does not fail the MoT but this year I am being told that it does fail. Is this right?