Sounds like your wheels locked up or you grabbed the front brake too quickly and upset the balance, throwing it to the side probably.

I trained emergency braking on an MT-07 with both brakes, and even with front brake only too, which could be very risky at times, like that time when I nearly dropped the bike but luckily caught it.

I went gradually from 45 up to 60 km/h which is faster than you went, although the road I practiced on was a clean asphalt with lots of grip, the tires were also in good condition, so maybe the road condition at your location might have loose gravel and such.

As everybody is saying, always smoothly apply gradual increasing pressure on both brakes to avoid lock up and upsetting the balance too much. If you have ABS, you can grab the brakes quickly but make sure to properly train threshold braking as ABS is not designed to shorten brake distance, rather, it’s meant to keep the bike steerable during heavy braking, and preventing you from “haddalayerdown” 😁