It's a "glitch" caused by the outdated tyre physics in GT. It is avoidable if you take certain driving measures as mentioned by Winnie, but this situation shouldn't occur in the first place if the game has proper tyre physics.
Basically, the tyre contact point with the track is simulated by a single point in GT, when in reality the contact patch is wider than that. So when this single point goes over the step of the kerb on exit, the sudden and large weight transfer upsets the car, which pitches it sideways, and then the tyre climbs back over the kerb, causing even more weight transfer and it's a death spiral.
In real life, because the contact patch is spread over a bigger area, the tyre "gradually" climbs over the step. No sudden weight transfer = everything controllable and you just feel a slight bump or a small correctable oversteer.
Assetto Competizione introduced a 5 point tyre model to fix this issue and Forza 8 is going to have an 8 point tyre model I believe. You can read in more detail here if you're interested:
https://www.assettocorsa.net/forum/...oducing-the-5-point-tyre-model-for-acc.59307/
Honestly it's embarrassing, but knowing PD they will just shoo it all under the carpet anyway. When the fans are frothing over the return of used car dealer and oil changes, who cares about physics right?