If the friction forces from the sway bar are greater than the friction between the tires and ground, when you go to make a turn, the trailer/tow vehicle junction (tongue) won't pivot and you'll be turning one LONG vehicle. If the friction difference isn't much, the trailer may pivot, but not enough and still not track straight.
Or, if conditions change it could have enough friction to the road to turn as you turn, but then if you hit a very low friction surface after the turn, the trailer can't straighten out. Not enough friction to the road surface to overcome the sway control.
Either case can be bad.
Now it does take a very low friction surface for that to happen, but that's what can happen with glare ice, hardpacked snow etc.
