Sounds like you're downshifting too early. Make sure you're in the right rpm range before downshifting. Although in some cases downshifting is used to help slow down, it isn't the main purpose - rely on the brakes mostly. The key reason to downshift is so that you're in the right gear for maximum acceleration OUT of the corner.
Downshifting too early.
Downshifting with improper use of the clutch.
Braking with too much steering at the same time.
Brake bias too far rearward.
Some combination of the above.
Beware that the lack of proper transmission and clutch modelling can make this confusing in rF2. Some cars will care that you are not using the clutch while others will not--and not based on reality, but whatever degree of settings and modelling went into the car.
100% easiest way is to move the brake bias forward, this always works well for me.
I usually find after a while I can start to move the bias more rearward as I get use to the car more and more but as a starting point having it forward a bit is always helpful.
It definitely seems key to have brake bias mapped to the steering wheel as well, comes in really handy to test changes as you are in track.
If it is present in all corners then you are probably just shifting down too early and thus upsetting the balance of the car. If it mostly happens at slow corners you may want to soften up the rear Anti Roll Bar. But go easy on it and make sure you are not just driving improperly before you start messing with the setup
or you might be braking to late because i play assetto corsa and this game and ive always noticed that you have to do all your braking before the corner and you cant get off too early