It makes perfect sense and what I was getting at when I mentioned to check if the fender was hitting the downtube.

If the wheel/forks can't center properly then it is pushed either left or right, then the geometry of the bike tends to want to correct for this and push the wheel back towards straight -just like a caster of a shopping cart (this is why you can ride no-handed or "ghost-ride" a bike without a rider on at all).

So the bike's geometry pushes the wheel straight but because something is hitting it can't STAY straight and goes too far to the other side just a tiny bit. Then the bike tends to push the wheel straight again. You feel this as an oscillation or wobble at the bars -just like when that shopping cart caster is wobbling around and can't stay straight.

This can also happen with a bad headset if they are binding up at one spot or annother and not able to move smoothly for minute automatic rake/trail geometry adjustments that the bike makes all by itself..