As mentioned, usually warped discs
You can usually see the distortion from heat discolouration on the high spots etc...
If they are ok, they may need a degrease - wipe them down and / or clean them off with a bit of emery cloth.
If the discs are ok, look to the surrounding areas such as head stock bearings, tyre condition and even the calipers / pads.

Breaking hard from speed - 80mph or more- on the RG makes the front get nasty to control, the wheel tries to lock up if you get too enthusiastic on the anchors.

Scary monsters.
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These aren't the droids you're looking for.